Molecular sensors confined on SiOx substrates

Accepted Manuscript Molecular sensors confined on SiOx substrates Vikram Singh, Prakash Chandra Mondal, Alok Kumar Singh, Michael Zharnikov PII: DOI: Reference:

S0010-8545(16)30281-8 http://dx.doi.org/10.1016/j.ccr.2016.09.015 CCR 112318

To appear in:

Coordination Chemistry Reviews

Received Date: Accepted Date:

11 July 2016 16 September 2016

Please cite this article as: V. Singh, P.C. Mondal, A.K. Singh, M. Zharnikov, Molecular sensors confined on SiOx substrates, Coordination Chemistry Reviews (2016), doi: http://dx.doi.org/10.1016/j.ccr.2016.09.015

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Molecular sensors confined on SiOx substrates

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Vikram Singh*,a Prakash Chandra Mondal,b Alok Kumar Singh,c Michael Zharnikov*,d

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a

Centre for Nanoscience and Nanotechnology, Panjab University, Chandigarh, India National Institute for Nanotechnology, University of Alberta, Edmonton, Canada c Department of Applied Chemistry, Babasaheb Bhimrao Ambedkar University, Lucknow, India d Applied Physical Chemistry, Heidelberg University, Heidelberg, Germany b

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Contents:

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2. Immobilization methodology, surface characterization and receptor design

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3. Ion sensors

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4. Gas sensors

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5. Sensors for (bio)molecules

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6. Sensors for explosives and warfare agents

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7. Conclusion and perspective

1. Introduction

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Acknowledgements

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References

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Abstract:

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The sensing of ions, ion-pairs, gases, toxic materials, biomolecules, and explosives represents

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an important issue of high practical relevance. Consequently, significant efforts are currently

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underway to develop sufficiently reliable and sensitive and yet possibly simple methods for

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this purpose. In this context, molecular sensing is particularly important, presenting itself as a

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useful tool for alleviating global security concerns, tackling environmental issues, and

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contributing to biomedical analysis and other associated areas. Innovatively designed

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molecules and metallo-organic moieties having a specifically tailored receptor site, a clearly

29

defined reporter group, and a signal processing unit are the basic components of a working

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molecular sensor. However, solution-based molecular sensors, although of great standing,

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suffer frequently from relatively long response time as well as from poor solubility and

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recyclability of the sensing material. In contrast, surface-confined sensors permit easier

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manoeuvrability in different media and are characterized by a shorter response time, better

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signal amplification, as well as by regeneration and recyclability of the sensor through 1

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chemical or physical post-treatments. Additionally, depending on the structural, redox, or

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optical behavior of the analyte and the sensing material, a suitable transduction technique or a

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combination of several such techniques may be employed for analyzing surface attributes and

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studying analyte-host interaction. In this regard, SiOx substrates and molecular assembles

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thereon offer distinct advantages over other common supports such as metals, polymers,

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membranes, etc. in terms of cost, durability, stability, and scope. In this context, this review

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deals with the most recent developments in design and fabrication of molecular sensors

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confined on SiOx substrates. Here we provide representative and relevant examples of

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molecular sensors on these substrates, arranged with respect to the entities under analysis,

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viz. ions, specific gases, biomolecules, explosives, and warfare agents, paying also some

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attention to the general aspects of such sensor design.

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1. Introduction

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Design of surface-confined molecular assemblies represents an exciting, multidisciplinary

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approach comprising synthetic chemistry, surface engineering, and nanotechnology [1-6].

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Fabrication of such assemblies on solid substrates can be performed in both ‘top-down’ (e.g.

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micropatterning techniques) and ‘bottom-up’ (e.g. self-assembly processes) fashion, with the

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latter methodology gaining larger popularity, leading to innovation in design and surface

53

characterization [7-13] as well as in molecular architectures with advanced functionalities

54

(see Figure 1) [1, 5, 14]. The respective molecular assembles have tailored, enhanced, and

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controlled physico-chemical, optical, and electrical properties. Accordingly, these assembles

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find applications in constructing molecular logic [15, 16], data storage media [17-20],

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molecular electronics [21-25], molecular switches [26-29], solar cells [30-32], therapeutics,

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and sensing devices [9, 33, 34].

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59 60 61 62

Figure 1: Schematic representation of various routes for the deposition and assembly of functional molecules on solid substrates. (A) monomolecular self-assembly, (B) layer-by-layer assembly, (C) spin coating method, (D) drop casting method (E) patterning of molecular and organic films, and (f) electrochemical deposition.

63 64

Sensors prepared on solid substrates exhibit almost exponential growth in popularity over the

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last years starting with the pivotal work of Rubinstein et al. where the authors utilized

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monomolecular coated gold electrodes for selective electrochemical sensing of Cu2+ [35].

67

Such sensor systems are superior to solution based sensing methodologies as they (i) provide

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an ordered, densely packed, and pre-organized sensing assembly leading to comparatively

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stronger and more specific interactions and stable host-analyte complexes and, therefore,

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generate fast and stronger responses; (ii) facilitate activity in different media; (iii) allow read-

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out signal amplification for higher sensitivity; (iv) are sensitive to very small amount of

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analyte (from micro to pico molar), and (v) enable reusability/recyclability [36]. Further,

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molecular assemblies on solid substrates are better suited for sensing purposes than such

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alternatives as membranes or polymers surfaces due to the lack of dispersal of analyte(s)

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through the membrane or polymer network [37, 38]. Additional advantages include

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generation of large active surface with only small amount of compound and almost negligible

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consumption of sensing material or analyte(s) [39].

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SiOx-based solid substrates, viz. single crystal silicon wafers, float glass, quartz, porous

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silicon, indium-tin oxide (ITO) coated glass, other specifically coated silicon supports as well 3

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as mesoporous silica and zeolites, represent a broad platform for molecular immobilization

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and assembly, in context of specific sensing activities. Alternatively, metal substrates,

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including gold in particular, have been rigorously employed for the preparation of

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monomolecular films useful, among other applications, for sensing purposes [6, 40, 41].

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Invariably and popularly, thiol-based coupling agents have been utilized for self-assembly on

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gold while silanes-based coupling agents have been primarily used for SiOx substrates.

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However, thiolate-anchored molecular assembles on gold substrates exhibit a limited

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physico-chemical stability, which originates mainly from soft-soft interactions, as compared

88

to siloxane-based monolayers on SiOx supports (250 vs. 450 kJ mol-1). In addition, due to the

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formation of strong covalent bonds, molecularly modified SiOx substrates can withstand high

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operational temperatures and have technological relevance in the semiconductor industry [42-

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44]. Also, SiOx substrates like glass and quartz allow optical read-out via UV-vis

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spectroscopy, fluorescence spectroscopy, and other optical techniques. Glass and quartz do

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not quench fluorescence and do not absorb light in the UV-vis spectral range (300-800 nm),

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being, therefore, optimal substrates for development of optical sensors, in contrast to, e.g.,

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metal substrates strongly quenching molecular fluorescence; for these surfaces facilitate

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plasmon excitation [45]. Other promising SiOx-based supports for sensor development are

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zeolites (in general, aluminosilicates) and mesoporous silica possessing a regular framework

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structure with tailored porosity and high surface area [46, 47]. These attributes also allow

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zeolites to be excellent molecular sieving agents. Finally, conductive and transparent ITO

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coated SiOx substrates, which have comparable conductive properties as metals, can also be

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used for spectro-electrochemical and electrochemical detection methodologies. However, on

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the other hand, fabrication of well-defined, functional monomolecular films on SiOx

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substrates is frequently difficult, suffering in particular from limited reproducibility; for they

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are highly sensitive to the chemical environment, particularly moisture [3, 14]. Further

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factors affecting the quality of the films and reproducibility of the preparation procedure are

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the parameters of the coupling layer, specific properties of the solvents, and possible side-

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reactions. Additionally, the complete monolayer might show only a limited photochemical

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stability in case of optical-based sensors on transparent SiOx substrates and limited

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electrochemical stability on conductive substrates. Nevertheless, in many cases, densely-

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packed and well-defined monolayers of functional molecules could be fabricated on SiOx

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substrates, exhibiting also sufficient overall stability and being suitable for a broad range of

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applications including sensor fabrication, as discussed in detail in recent reviews by

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Reinhoudt [4] and Schubert [14] groups. 4

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In this context, our group has explored the engineering of SiOx substrates for immobilization

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of Cu(II), Ru(II), Os(II) and Fe(II)-polypyridyl complexes, their dyads (bilayer), triads

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(trilayer), as well as hetero- and homo-dinuclear complexes for applications stretching from

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catalysis, electro-chromism, opto-electronics, molecular memory, photonuclease activity to

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sensing [20, 48-56]. These studies are only a part of general efforts towards the use of SiOx

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substrates as a platform for creation of useful molecular systems, also in the framework of

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sensing, which is the subject of this review. In the following sections, we will briefly review

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the ideas behind the design of molecular receptors, respective immobilization strategies, and

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surface characterization techniques, which have been employed to monitor the behavior and

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performance of surface-attached receptors. Then, we will focus on the utility of molecular

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films on SiOx surfaces for sensing a broad range of ionic species, gases, (bio)molecules,

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explosives as well as chemical and biological warfare agents.

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2. Immobilization methodology, surface characterization and receptor design:

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SiOx substrates are characterized by silanol number (αOH or NOH), that is, surface

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concentration of –OH groups, which forms an important criterion for achieving the ‘complete

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monolayer’ coverage on these supports. In addition, the density and distribution of silanol

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groups can vary on different substrates and are also subjected to physicochemical conditions

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like temperature, pressure and moisture. For instance, NOH is ~ 5.0 nm-2 for amorphous silicas

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including silica gels, aerosilogels, and porous glasses at 180-200 oC [57], while it is estimated

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at ~3.3 nm-2 for crystalline mesoporous silica FSM-16 [58]. Furthermore, two types of silanol

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groups are present at the silica-aqueous interface, viz. isolated (pKa = 4.9; 19% concentration)

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and hydrogen-bonded (pKa = 8.5; 81% concentration) silanol groups with varying local

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surface distributions. Importantly, when the neighbouring silanol groups form a H-bond to

139

each other (~46%), the distance between them is less than 3.3 Å. However, when H-bonding

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between these groups is mediated by water, the respective distance increases to 3.5-5.5 Å

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[59].

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Figure 2: Schematic representation of different kinds of silanol groups on planar silica surfaces. (A) isolated silanol groups (19%); (B) water bridged silanol groups (35%); (C) silanol groups directly H-bonded to the adjacent groups (46%).

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Fabrication of molecular sensors on SiOx substrates is a stepwise process (see Figure 2) and

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includes, as the first step, surface-activation - for cleaning and maximizing the density of the

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silanol groups on the surface, thereby making the surface hydrophilic and hence, susceptible

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to hydration [57]. Surface-activation can be performed either by employing strong acids or

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oxygen plasma as well as by sonication. In the second step, silane-based attachment

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chemistry (silanization of surfaces), which is believed to be the most dependable approach, is

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employed for generating thermally, temporally, and chemically stable coupling layers,

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serving as templates for subsequent modification and/or attachment of the sensor units [13].

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Such coupling layers allow covalent linkage to the SiOx substrate through their specific

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anchoring groups (e.g. aliphatic or aromatic trichlorosilanes and trialkoxysilanes) while the

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packing efficiency and monolayer organization are mostly determined by the spacer units of

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the molecules comprising these layers (see Figure 2). The exact mechanism of assembly of

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coupling layers, however, depends on a number of factors, for instance, type of substrate,

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surface water content, solvent used, temperature, formation/deposition time and, therefore, is

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still a subject of controversial discussions [4, 60, 61]. The prevailing opinion is that, for

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trialkoxysilanes, hydrolysis of the alkoxy groups takes place and the resulting three hydroxyl

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groups per silane-bearing molecule react with the silanol-terminated surface to form siloxane

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linkages. However, in general, the attachment of the coupling layer to the surface is mediated

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by only one or two hydroxyl groups. The residual, unbound hydroxyl group(s) could lead to

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polymerization and surface heterogeneity and, therefore, various approaches including

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postsilanization curing, creation of suitable anhydrous conditions, and others were devised to

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avoid or at least to diminish these constraints [62].

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In the final step, the terminal functional groups attached to the spacer units and comprising

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the coupling layer-ambient interface can be chemically modified or used directly as selective

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docking sites to generate/attach specific moieties mediating the desired sensor/biosensor

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behavior of the entire assembly. Most significantly, chemical modifications of the terminal

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groups via surface reactions can effectively allow to tune the surface characteristics such as

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conductivity, wetting properties, adhesion ability, friction and other attributes, in a desired

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and controlled manner [63-65]. These chemical modifications and the attachment steps are

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generally performed through nucleophilic substitution reactions, click chemistry, Diels-Alder

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reactions, coordination chemistry, C−C coupling reactions, electrostatic interactions, or

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supramolecular chemistry (see Figures 3 and 4); in most cases they can be carried out by

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simply immersing the functionalized substrates into a dilute solution of receptor molecules

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for a specific time at a suitable temperature [34]. Thereafter, such standard procedures as

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washing with suitable solvents, sonication, and drying under inert gas are usually performed,

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resulting in ensemble of uniquely designed receptor moieties immobilized on SiOx substrates.

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Additionally, it is also possible to fabricate mixed monolayers, as well as homogeneous and

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heterogeneous dyads, triads, and oligomers using the attachment methodology in a successive

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fashion (layer-by-layer) [54-56]. Therefore, this general approach represents a powerful yet

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simple strategy to fabricate a highly dense, stable, organized, and defect-free oriented

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assembly of receptor moieties covalently bonded to desired SiOx substrate via a coupling

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layer.

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Figure 3: Schematic representation of monolayer attachment chemistry on activated SiOx substrate(s) (with maximal possible density of the silanol groups) where the terminal group of a coupling layer can be chemically modified or used as a

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190 191

specific binding site to attach the sensor moiety onto the substrate. Such functionalized substrates can be applied in diverse areas depending on the kind of the terminal group and the attached moiety.

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Figure 4: Representative examples of chemical modification of the terminal groups of the coupling layer and their reactions with the functional moieties. (A) click chemistry, (B) supramolecular modifications, (C) metal-ligand interaction, and (D) nucleophilic substitution.

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Further, it is imperative to analyze the functionalized substrates to have a clear understanding

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of the properties of molecular films, viz. wettability, thickness, packing density, orientation,

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available functional groups, etc.. In view of a small amount of the analyzed material (from

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micro to picomol per cm2) and general complexity of the systems, a combination of highly

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sensitive surface analysis techniques must be usually employed to get an insight into the

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surface properties, generally involving electrons, photons, X-rays, neutral species, or ions as

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a probe beam, which interacts with the system in one way or the other and is analyzed

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afterwards (see table 1). Some techniques provide information by measuring changes in

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energy/intensity of the primary beam while others analyze secondary ejected moieties from

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the sample upon the excitation provided by the probe beam. Some other techniques involve

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mechanical contact between the probe and supported molecular film. For instance, as an

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example of monitoring processes at surfaces and interfaces, for the surface reaction in Figure

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4A, surface-enhanced IR reflection spectroscopy clearly demonstrated the disappearance of

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azide (N3) absorption peak at ~2100 cm-1 in the second step, thereby confirming 1,3-dipolar

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cycloaddition on Br-terminated monolayers. Further, in the reaction sequences in Figure 4B,

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–CN absorption (~2246 cm-1) completely disappeared in the FT-IR spectrum in the first step; 8

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in the second step, ellipsometry characterization recorded an increase in the height of the

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monolayer by 0.4 nm (–NH2 to –NCS transformation) and; in the third step, contact angle

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measurements revealed change in polarity of the surface (contact angle changed from 49o to

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68o) with the covalent immobilization of the cyclodextrin unit. In the reaction shown in

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Figure 4C, XPS clearly demonstrated two well-resolved components related to the presence

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of pyridine (N) and pyridinium (N+) at 399.2 eV and 402 eV, respectively with N/N+ = 2.8.

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Note, however, that detailed description of surface analytical techniques is beyond the scope

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of this review, therefore, readers are advised to look for other comprehensive articles/reviews

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(see e.g. refs 7, 9, and 66). Surface characterization techniques

Provided Information

Contact Angle Goniometry Quartz Crystal Microbalance (QCM)/ Surface Acoustic Waves (SAW) ATR-IR Spectroscopy UV-vis Spectroscopy Fluorescence Spectroscopy Ellipsometry Surface Plasmon Resonance (SPR) X-ray Photoelectron Spectroscopy (XPS) Auger Electron Spectroscopy (AES) Ion Scattering Spectroscopy (ISS) X-Ray Reflectivity (XRR) Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy Secondary Ion Mass Spectrometry (SIMS) Atomic Force Microscopy (AFM) Scanning Tunneling Microscopy (STM) Scanning Electron Microscopy (SEM) Transmission Electron Microscopy (TEM) X-Ray Diffraction (XRD) Low Energy Electron Diffraction (LEED) Cyclic Voltammetry (CV) Impedance Spectroscopy

Wetting properties, roughness Change in mass upon deposition Functional groups, orientation Packing density, molecular footprint Packing density, molecular footprint Film thickness; refraction index Film thickness Elemental composition Elemental composition Elemental composition Thickness, roughness Chemical identity, molecular orientation Molecular composition Morphology, structure, packing Morphology, packing, structure Imaging, grain structure Atomic structure, chemical bonding Crystalline phases, strain, thickness Microstructure, order, periodicity Surface coverage, defects, thickness, kinetics Surface coverage, defects, thickness, kinetics

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Table 1: Most common surface analysis techniques. The list is not exhaustive.

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To detect a particular molecular entity in gas or solution phase, an appropriate receptor/sensor

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unit must be designed which has some property complementary to the entity being ‘sensed’.

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Such a property can be either structural, physical or chemical attributes or the combination

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thereof [67-70]. In particular, for detection of charged species, supramolecular host-guest

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interactions are highly effective, which mainly exploit the size/charge of the charged species.

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For this purpose, receptors like calixarenes, catenanes, crown ethers, amide-based moieties,

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rotaxanes, and others have been designed, with continuous improvements in their physico-

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chemical properties by means of appending fluorescent tags, redox active substituents, and

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chromophore groups for enhanced electronic communication [71-76]. Recently, metal-

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organic complexes have also been applied as receptors; for they possess outstanding photo-

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physical and redox properties [76-78]. Weak interactions like van der Waals forces (<5

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kJ/mol but variable depending on the intermolecular spacing), hydrogen bonding (4-120

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kJ/mol), dipole-dipole (5-50kJ/mol), dipole-ion (50-200 kJ/mol), ion-ion (200-300 kJ/mol),

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π-π interactions (0-50 kJ/mol), cation-π interactions (5-80 kJ/mol) and others as well as

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electron-transfer based changes of redox state and a strong covalent linkage between the

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analyte and receptor form the basis of the readable output signal in the form of change in the

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structural, physicochemical or electrochemical attributes of the receptor-analyte couple. The

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respective interactions at the monolayer-analyte interface are considered to be much stronger

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and different from those at the liquid-liquid or liquid-gas interfaces as represented by ‘bulk’

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phenomenon [36]. In addition, the spacer length and its hydrophobicity/hydrophilicity also

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affect the sensitivity and selectivity of the sensing process (so called “spacer layer screening

244

effect”) [79]. Besides, in case of conjugated polymers (CPs) based sensors, presence of a

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suitable side-chain can also affect the photophysical properties of the sensor, providing

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additional means to improve its performance [79-81]. The output signals/sensory response

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relies upon a specific molecular receptor property under the observation which will be

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affected by the interaction with analyte(s). The respective effect can be e.g. fluorescence

249

enhancement/quenching, shifts in absorptions bands, appearance/disappearance of specific

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bands in the UV-vis or NIR spectra, change in a redox state (electrochemistry), or different

251

impedance value. Further, the processes at the monolayer-analyte interface can be monitored

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by using such standard techniques like infrared spectroscopy (changes involving functional

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groups); XPS (changes in elemental composition); QCMs/SAWs (changes in mass); AFM

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(changes in surface morphology), and others.

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In the following sections, we will present and discuss most representative (in our opinion)

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examples of sensing of charged species (i.e. ions), gases, and practically relevant

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(bio)molecules, as well as explosives and warfare agents by siloxane-based molecular sensors

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on SiOx supports. We will not specifically discuss details of the surface characterization but

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put emphasis on design and performance of these sensors.

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3. Ion sensors:

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Ions are ubiquitous in nature and are fundamental to a number of chemical, environmental,

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medical, and biological processes. For instance, Na+/K+ pump, Fe2+/3+ in hemoglobin, Cl- in

264

extracellular fluids, F- for healthy bones and teeth growth are representative of the importance

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of ions to living organisms. It is well known that excess/deficiency of these ions can result in

266

a serious illness or even be fatal. Alternatively, ions like Cr3+/6+, As3+, TcO4-, an excess of

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NO3-/SO42- and others are potentially damaging to the environment and dangerous for living

268

organisms. Therefore, ion sensing and monitoring is crucially important in spite of difficulties

269

in their recognition, particularly that of anions, because of varying geometries and associated

270

intrinsic properties [82]. Herein, we will review most important (in our opinion), ‘state-of-

271

the-art’ monolayer-based ion sensors.

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Several groups have been working on selective and sensitive sensing of ions using differently

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functionalized SiOx substrates. Among other systems, surface-attached metal-polypyridyl

274

complexes were extensively utilized, particularly polypyridyl complexes of osmium (Osbp)

275

and ruthenium (Rubp). In one such example, Ruiter et al. performed optical recognition and

276

quantification of ppm levels of Cr6+ in aqueous and organic media using Os(II)-bipyridyl

277

(Osbp2+) monolayer on glass with ~1 min exposure time (Figure 5) [83]. The sensing process

278

involved electron transfer chemistry in the monolayer and was pH dependent with the

279

oxidation rate higher at pH = 0.3 and somewhat lower at pH = 7.5. The analyte-induced

280

changes in the redox state of the receptor molecules were manifested by the disappearance of

281

the singlet and triplet metal-to-ligand charge-transfer (MLCT) bands at λmax = 516 and 692

282

nm, respectively, and hence, could be easily monitored by conventional UV-vis spectroscopy.

283

The immobilized complex was inert towards a broad variety of different metal ions, being

284

highly selective for Cr6+ under the conditions of the test experiments. Interestingly, the

285

analogous Osbp2+ monolayer was further used for the detection of Fe3+ in H2O and CH3CN

286

by Gupta et al. (Figure 5) [84]. These authors demonstrated that the given dual sensor system

287

is capable of detecting either of Fe3+ or Cr6+ depending on pH value. It was observed that at

288

similar concentrations of Fe3+ and Cr6+, response towards Cr6+ is 6-times larger within ~1 min

289

of exposure time. However, in the mixture of these ions, Fe3+ can be selectively sequestered

290

by strong base treatment. In another study, Osbp2+ monolayer has been utilized to detect NO+

291

in THF and CH3CN with complete oxidation within 10-60 min depending on NO+

292

concentration (Figure 5) [85]. The process involved the similar redox chemistry as in the

293

examples above and allowed an easy detection of NO+ within the 0.36-116 ppm

294

concentration range. Significantly, the sensing monolayer could be easily reset using H2O

295

with no perceptible hysteresis (Figure 5). In addition, when the sensing Osbp2+ monolayer

296

was screened against potential interfering ions like NH4 +, tBu 4N+, Et4N+, K+, Na+, and Ca2+,

11

297

no perturbations in its UV-vis spectrum were detected, with the character of solvent making

298

no effect as well.

299 300 301

Figure 5: Schematic of the versatile redox chemistry for the Os(II)-bipyridyl monolayer (Osbp2+) on glass. Changes in the sensing conditions, viz., solvent, pH, etc. allow selective sensing of different cations, viz., Cr6+, Fe3+, NO+ [83-85].

302

Similar to the above methodology, we reported ppb-level Fe2+ sensor on the basis of a

303

covalently immobilized Ru(II)-bipyridyl (Rubp2+) monolayer on the soda-lime glass (Figure

304

6) [86]. The sensing monolayer was first treated for 3 min with Ce4+ in water, resulting in the

305

Ru(II)
Ru(III) oxidation, which was monitored by UV-vis spectroscopy showing a gradual

306

disappearance of the MLCT band at λmax = 463 nm. However, this band reappeared within ~2

307

min when the oxidized monolayer was treated with 1.0 ppm of Fe2+ corresponding to the

308

Ru(III)
Ru(II) process (Figure 5). The sensitivity limit for the respective Fe2+ detection was

309

estimated at 5 ppb. An additional important implication of the above studies is the potential

310

usefulness of the ion-triggered electron transfer processes for design of complex molecular

311

systems such as mixed monolayers with two distinct oxidation states for such applications as

312

creation of molecular logic and molecular memory [83, 87-89].

313 314

Figure 6: Schematic of a Ru(II)-bipyridyl (Rubp2+) monolayer on glass used for ppb level detection of Fe2+ [ 86].

12

315

Not only monolayers but redox-active multilayer(s) have also been used for ion sensing

316

purposes. For instance, we fabricated terpyridyl-based heterometallic molecular triads on

317

glass substrate(s) and utilized this assembly for the detection of NO+ in acetonitrile [55]. In a

318

multimetallic-multilayer system (Os-PT/Cu/Ru-PT/Cu/Fe-PT) involving metal-(pytpy)2⋅2PF6

319

(metal-PT) complexes with [pytpy = 4’-(4-pyridyl)-2,2’:6’,2’’-terpyridyl], NO+ exposure

320

produced selective oxidation of the Fe2+/Os2+ centers, while Ru 2+ did not oxidize (Figure 7),

321

as monitored by UV–vis spectroscopy. According to the experimental data, the MLCT band

322

at λmax = 577 nm (Fe-PT) showed a bathochromic shift (change in wavenumber = 583 cm-1)

323

along with ~60% reduction in absorbance within 30s of NO+ exposure. These changes of the

324

UV-vis spectra of the triad film were attributed to the successive quaternization of the

325

terminal pyridyl group and oxidation of Os2+/Fe2+ to Os3+/Fe3+. Significantly, successive

326

exposure of the triad film to H2O and Et3N restored the initial redox state of the metal centers

327

(Figure 7), thereby regenerating the sensor. Interestingly, these chemical transformations

328

were not only relevant in context of the sensing applications but also facilitated design of

329

three- and four-input molecular logic gates and circuits [1, 15, 90].

330 331 332 333 334

Figure 7: Schematic representation of a redox-active metal-complex triad assembly on SiOx substrate. Exposure to NO+ oxidizes/quaternizes the terminal pyridine N-atom of the triad assembly while a further exposure oxidizes Os2+
Os3+ and Fe2+
Fe3+ with concomitant changes in the UV-vis spectra of the triad (from left to right). The system can be regenerated by successive exposure to H2 O and Et3N (from right to left) [55].

13

335

Not only molecular films but also conjugated polymers (CP) like phosphonate-functionalized

336

polyfluorene have been employed for solution-based detection of metal ions such as Fe3+. On

337

cue, Wu et al. immobilized the above-mentioned CP on a glass substrate via (3-aminopropyl)

338

triethoxysilane as coupling agent (Figure 8A) and found only minimal variations in the

339

photoluminescence (PL) properties between the CP in solution and immobilized on glass

340

[91]. However, the latter system was extremely sensitive to Fe3+. In particular, only 5µM of

341

Fe3+ in THF was able to quench PL (monitored at λmax ≈ 412 nm) of the immobilized CP

342

within 1 min under the standard conditions while potential interfering analytes (5µM each of

343

Ni2+, Cu 2+, Co2+, Co2+, Hg2+, Zn2+, Fe2+, Li+, Sr2+, Ca2+, Pb2+, Ag+, K+, Mg2+, and Cd2+) did

344

not induce any substantial PL quenching. The Fe3+ detection limit was estimated at ca. ~8.4

345

ppb. When tested in an aqueous medium, the PL quenching was observed as well but to a

346

lesser extent than for THF. Importantly, the sensor could be easily regenerated for repeated

347

use using dilute NH3. In another report, where rhodamine-appended polyterthiophene

348

polymeric material was immobilized as thin films on ITO-coated glass substrates for sensing

349

purposes, Kaewtong et al. reported a good selectivity of this film to Hg2+ (Figure 8B) [92].

350

Complementary potentiometric experiments showed that exposure to Hg2+ decreases the

351

potentials of these conducting films, with response time as short as 30s and detection limit

352

approaching 0.10 µM. The authors attributed the changes to either improved charge carrier

353

transport properties or reduction in doping states through ion–ion interactions, or perturbation

354

in π−π interactions. Favorably, rinsing with ethylenediaminetetraacetic acid (EDTA) solution

355

regenerated the polymeric film to the original state, making the sensor reusable.

356

14

357 358 359 360

Figure 8: Schematic of (A) immobilization of phosphonate functionalized-polyfluorene on a glass substrate and its use in the reversible detection of Fe3+ based on fluorescence quenching of the host upon the interaction with Fe3+ [91]; (B) rhodamine-appended polyterthiophene thin film on ITO substrate for detection of Hg2+, where the presence of Hg2+ reduced the potential of these films within 30s [92].

361

As one of the first examples of harnessing of supramolecular interactions at the monolayer-

362

analyte interface, van der Veen et al. reported covalent immobilization of a pyrene-appended

363

calix[4]arene on a glass substrate and used it for fluorescence-based selective detection of

364

Na+ (Figure 9A) [93]. The calix[4]arene moieties were attached covalently to the substrate

365

via 3-aminopropyltriethoxysilane as the coupling reagent. When the monolayer was exposed

366

to Na+ solution in methanol, the intensity of the pyrene monomer fluorescence at λ max = 385

367

and 395 nm decreased while the emission corresponding to the pyrene excimer (λmax = 480

368

nm) increased in intensity. At the same time, such potentially interferring ions as K+ and Cs+

369

did not affect the emission spectrum of the monolayer under the analogous experimental

370

conditions. In a subsequent work, the same group introduced a combinatorial approach to

371

fluorescent-monolayer-based sensing of cations/anions in both aqueous and organic media

372

(Figure 9B) [94-96]. Within this approach, this group created libraries of simple fluorescent

373

molecules attached to glass substrates which are not, in general, specific to one particular ion

374

but are responsive to several ions through non-specific interactions. According to the authors,

375

this combinatorial approach can allow to avoid complicated receptor design and is completely

376

transferable to microscale via µ-contact printing and lithography techniques [97, 98].

15

377 378 379 380 381 382

Figure 9: (A) Schematic of pyrene-appended calix[4]arene covalently immobilized on a glass substrate for selective detection of Na+ in methanol [93]. (B) Schematic illustration of how an analyte (yellow) can coordinate with the molecular assembly on a glass substrate with the fluorophore group reporting the fluorescence changes [94]. An array of such fluorescent sensors can detect cations/anions in a combinatorial manner, by changing the fluorophore or the associated chemical functionalities.

383

Following a similar supramolecular design strategy, Gulino et al. fabricated a monolayer of

384

pyridylazacalix[4]arene on a quartz substrate, which could reversibly sense Li+ even in the

385

presence of highly competitive Na+ (Figure 10A) [99]. The sensing process was monitored

386

using UV-vis spectroscopy and XPS. In the UV-vis experiments, the monolayer, with trapped

387

Na+ ions, was exposed to increasing concentrations of Li+ (2.5-33.0 ppm), resulting in

388

opposite hypochromic and hyperchromic effects for the optical bands centered at λ = 421.3

389

and 572.7 nm, respectively. This behavior was attributed to displacement of Na+ by Li+ ions

390

which was ably supported by the XPS spectra where the Na 2s emission at 64.3 eV gradually

391

decreased in intensity accompanied by the concomitant appearance of the Li 1s emission at

392

56.4 eV upon successive exposure of the monolayer to CH3CN solutions of increasing Li+

393

concentration. Finally, at high concentrations of Li+, the Li 1s emission became the only

394

signal detected in the 50–70 eV binding energy range. Interestingly, the ion displacement

395

could even be viewed using the naked eye as immersion of the quartz substrate engineered

396

with the sensing monolayer into Li+ solution tainted the sensor pink while subsequent

397

priming with hydrochloric acid regenerated the original color.

16

398

In another study, Lupo et al. also utilized supramolecular host-guest interactions for

399

recognizing polyatomic cations such as n-butylammonium ion (Bu4N+). For this purpose the

400

authors functionalized quartz substrate with (p-chloromethyl)phenyltrichlorosilane as the

401

coupling layer and then covalently attached calix[5]arene having a 12-aminododecyl moiety

402

at the lower rim, thereby forming a calixarene-based monolayer (Figure 10B) [100]. A

403

sensing of Bu4N+ at ppm levels as well as detection of a biologically relevant analyte such as

404

1,5-pentanediammonium ion (cadaverine.2H+) after 1 min immersion of the monolayer in

405

CH3CN solution containing these ions could then be performed optically, using UV-vis

406

spectroscopy. The detection was based on the appearance of a new band at λmax = 359 nm in

407

the UV-vis spectrum of the monolayer, while rinsing with THF for 1 min restored its original

408

state for repeated use, which was tested up to 8 cycles.

409

As a continuation of the above work, the same group appended pyrene to calix[5]arene

410

monolayer (Figure 10C) [101]. Inclusion of the pyrene moiety into the calix[5]arene structure

411

led to the appearance of absorption bands in the UV-vis region (λmax = 347 nm and related

412

shoulders at λmax = 330 and 318 nm) and emission bands in the fluorescence spectrum of

413

calix[5]arene. Note that calix[5]arene alone did not show any such characteristics. It was then

414

observed that the above, pyrene-substituted SAMs exhibit conformational/geometrical

415

changes when exposed to n-alkylammonium ions. These structural alterations were

416

accompanied by certain changes in the electronic structure and generated “emission

417

variations”, which enabled detection of the analytes.

418 419 420

Figure 10: Schematic of a calixarene-based monolayers on quartz substrates suitable for (A) Li+ detection even in the presence of high concentration of Na+ [99] and (B and C) sensing of n-alkylammonium cations [100, 101].

17

421

For selective recognition of Cu2+ ions, a monolayer of N-(4-hydroxyphenylethyl)-4,5-di[(2-

422

picolyl)amino]-1,8-naphthalimide on quartz substrate was fabricated (Figure 11A) [102].

423

When the monolayer was exposed to solutions with different concentrations of Cu2+, distinct

424

changes in its UV-vis spectrum were recorded, with a saturation behavior at above 0.1 ppm

425

of Cu2+. This sensor was Cu 2+ specific as 0.1 ppm aqueous solutions of Na+, K+, Mg2+, Zn2+,

426

Cd+, Hg+, or Ca2+ did not generate a similar optical response. This behavior was also

427

corroborated by complementary XPS measurements, where a characteristic Cu 2p3/2,1/2

428

doublet at 933.1 and 952.9 eV appeared after only 1 min of Cu2+ treatment. Moreover, the

429

absence of shakeup satellites for this doublet confirmed the presence of Cu2+ ions in the

430

monolayer. Favorably, treating the monolayer with EDTA solution regenerated it for

431

potential reuse. Analogous complexation strategy was also utilized by Ju et al. who

432

demonstrated the use of specifically designed rhodamine-based monolayers on a glass

433

substrate for fluorescence-based selective detection of Pb2+, one of the heavy metal ions

434

which pose a serious health and environment hazard (Figure 11B) [103]. The sensor

435

fabrication

436

(NHS) via amine-NHS coupling, leading to a tripodal structure. The rhodamine moieties

437

(spirolactam form) in the monolayer served as the signaling units while the alkyl-amine

438

spacers allowed facile Pb2+ binding. The monolayer was exposed to a variety of CH3CN

439

solution of perchlorate salts with Li+, Na+, K+, Rb +, Cs+, Mg2+, Ca2+, Sr2+, Ba2+, Al3+, In3+,

440

Fe2+, Fe3+, Co2+, Cu2+, Ag+, Zn2+, Cd2+, Hg2+, and Pb2+ cations. An enhancement of

441

fluorescence intensity, appearing as the transformation from colorless to red as observed via

442

fluorescence microscopy, occurred only for Pb 2+. This behavior was ascribed to the formation

443

of a Pb 2+ complex which led to opening up of the spirolactam, thereby, resulting in

444

enhancement of fluorescence intensity.

included

modification

of

glass

substrate

by N-hydroxyl

succinimidyl

18

445 446 447 448

Figure 11: (A) Schematic of a monolayer of N-(4-hydroxyphenethyl)-4,5-di[(2-picolyl)amino]-1,8-naphthalimide on quartz substrate serving for reversible detection of Cu2+ using UV-vis spectroscopy [102]. (B) Schematic of rhodamine (spirolactam form) -based monolayers for fluorescence-based reversible and selective detection of Pb2+ [103].

449

For sensing of Cd2+, which is another widely studied, toxic cation, Bronson et al. fabricated a

450

thin film and a monolayer combining the 5-Chloro-8-methoxyquinoline and diaza-18-crown-

451

6 moieties (1) on a quartz substrate (Figure 12) [104]. When (1) was exposed to only 100 nM

452

of Cd2+, an increase in the fluorescence intensity was observed within 1 min, with a

453

comparably larger effect for the thin film. It was observed that Cd2+ formed a stable complex

454

(log Ka = 6.1) with (1). The sensor could be used reversibly although a gradual loss of

455

fluorescence intensity of (1) was observed with each cycle. The sensing process was not

456

affected by the presence of such potentially interferring ions as Na+, K+, Ca2+, Zn2+ but was

457

noticeably distorted by the presence of Cu2+ which could strongly quench the fluorescence of

458

(1) even at the large excess of Cd2+. Consequently, sensor array based strategies were

459

discussed to have clear differentiation between the concurring analytes.

460 461 462

Figure 12: Schematic of 5-Chloro-8-methoxyquinoline and diaza-18-crown-6 based monolayer/thin film on quartz substrate serving for selective sensing of Cd2+ [104].

19

463 464

In another study, where mesoporous silica material was used as a sensing platform, Lin et al.

465

developed a Cs+ specific sensor, which is contained in the liquid waste of nuclear reactors

466

(radiocesium) [105]. At first, the authors functionalized mesoprous silica with ethylene

467

diamine (EDA) terminated [1-(2-aminoethyl)-3-aminopropyl]trimethoxysilane coupling

468

agent and then immobilized Cu(II) ions via the strong chelating effect of EDA. Finally, they

469

immobilized hexacyanoferrate(II) moiety (as sodium or potassium salt; strong ion exchanger)

470

to build the overall sensing unit (Figure 13). XPS provided Na1.3Cu 1.5Fe(CN)6 as the

471

composition of copper ferrocyanide on the given substrate and

29

Si solid state NMR based

2

472

evaluation gave 4.9 silanes/nm as coupling layer density. The sensor could detect/sequester

473

at least 2 ppm levels of Cs+. XPS data revealed that the Na+ (or K+) ions in the assembled

474

sensing units were removed almost completely upon the sensor exposure to Cs+, with a Cs/Fe

475

ratio close to 1.4, similar to the original Na/Fe ratio.

476 477

Figure 13: Representation of Cs+ specific sensor functionlaization over mesoporous silica [105].

478

Within a different approach to cation sensing, methods have also been devised to selectively

479

detect organic/inorganic metal salts in contrast to inorganic ones. As a representative

480

example, Lu et al. developed an innovative way to selectively detect organic salts of Cu2+

481

(copper acetate and copper propionate) in contrast to the respective inorganic salts as well as

482

to organic salts of other metal ions in water by monitoring fluorescence from the sensing

483

monolayer [106]. The monolayer had 3-glycidoxypropyltrimethoxysilane as a coupling agent

484

and was terminated with pyrene moiety (Figure 14A). Static quenching of the emission band

485

centered at λ = 500 nm was observed upon the exposure to organic copper salts, with Cu 2+

486

acting as quencher (by binding to the free imino group) while the acetate/propionate groups

487

promoted an easy access of Cu 2+ through the hydrophobic spacer layer, which was not

488

possible for the inorganic salts. Significantly, the sensor could be quickly regenerated by

489

repeated washing with aqueous EDTA solution. The role of the spacer moieties in the

490

detection process was further highlighted by the use of the same spacer group for the 20

491

preparation of an anthryl-terminated monolayer on glass substrates (Figure 14B) [107]. The

492

flexibility and hydrophobic nature of the long spacer group allowed the creation of relatively

493

hydrophobic microstructures in aqueous solutions, with the anthryl moiety staying inside

494

these microstructures. Therefore, organic copper salts could quench the fluorescence in an

495

aqueous medium as the organic anions could penetrate into the microstructure. When the

496

medium was changed from aqueous to methanol, the fluorescence could also be quenched by

497

inorganic copper salts.

498 499 500 501

Figure 14: Schematic of the monolayers prepared by the Fang group on glass/quartz substrates for selective detection of inorganic and organic metal salts by monitoring fluorescence response from the monolayers upon their exposure to the salts. The role of the spacer group in the detection process was of a great significance [106-110].

502

Complementary, for detecting inorganic metal salts, another fluorescence-based sensor was

503

developed, having hydrophilic oligo(oxoethylene) spacer group and pyrene-moiety

504

termination (Figure 14C) [108]. Quenching of fluorescence was observed at only 5µM of

505

inorganic salts of Cu2+ and Hg2+ as they form complexes with amino and oxoethylene groups

506

in the spacer. The detection limits were ~4.0 × 10−7 M and ~3.0 × 10−7 M for Cu2+ and Hg2+,

507

respectively. In another approach, the introduction of sulfonyl groups connecting the pyrene

508

moieties and the spacers additionally decorated with amino groups allowed a creation of a

509

hydrophilic monolayer which was capable of selective detection of inorganic Cu2+ salts in

510

contrast to the respective organic compounds (Figure 14D) [109]. Within a further study by

511

the same group, a novel dansyl-functionalized monolayer was designed (Figure 14E); it

512

showed cross-reactive fluorescence responses to the presence of Cu 2+ and Hg2+ in H2O,

513

CH3CN or THF solutions. This resulted in a distinct recognition pattern for these metal ions

514

at their particular concentrations and allowed their clear discrimination [110].

21

515 516

4. Gas sensors:

517

The atmospheric air contains a broad variety of chemical species which are either present

518

naturally or as a result of industrial, commercial or household activities. Some of these

519

species like O2 and atmospheric water, which are required for sustained life, must be present

520

in adequate amounts. Others, like toxic gases, offensive odors or volatile organic compounds

521

(VOCs), should be below the designated limits. Additionally, burning of fuels or their

522

explosion can pollute the environment as well as pose a grave danger to living organisms.

523

Under these circumstances, constant monitoring of atmospheric composition becomes

524

inevitable and therein lies the importance of gas sensors, along with numerous industrial and

525

scientific applications. Herein, we will review most representative SiOx based monolayer-like

526

gas sensors.

527

Gulino et al. reported a covalent assembly of bimetallic rhodium complexes on glass

528

substrates using (p-chloromethyl)phenyltrichlorosilane as the coupling agent for ppm level

529

detection of carbon monoxide (CO) using UV-vis spectroscopy transduction technique

530

(Figure 15A) [39]. The interaction of CO with the monolayer resulted in enhancement of the

531

ligand-to-metal charge-transfer (LMCT) band (λmax = ~300 nm) and appearance of a low-

532

intense band (λmax = 560 nm) associated with the formation of strong metal-carbonyl bond

533

(Rh−CO). The developed sensor was highly selective to CO as it did not respond to the

534

presence of potentially interferring N2, N2O, O2, NOx, CO2, CH4, Ar, H2, or their mixture in

535

the probed gas. Moreover, the sensor could be easily regenerated by a gentle heating or

536

exposure to an inert gas which removes CO through a well-known dissociative mechanism.

537

Within an alternative approach, Lupo et al. assembled a monolayer of Ru(II)-bipyridyl

538

complexes on a quartz substrate, which, upon the excitation at 480 nm, showed noticeable

539

photoluminescence (PL) signal in the range from 600 to 850 nm with prominent triplet charge

540

transfer bands at λ = 680, 720, and 760 nm (Figure 15B) [111]. The monolayer was sealed in

541

a quartz cuvette and PL was monitored while maintaining either N2 or O2/air atmosphere for

542

at least ~30 min. The presence of N2 did not affect the PL intensity but the presence of O2

543

quenched the PL to some extent, with a certain PL signal being still observed. The PL in

544

air/O2 atmosphere was then considered as a reference and the influence of other potentially

545

interfering gases, viz. CO, CH4, H2, Ar, NOx, and N2 was measured by leaking them

546

individually for 1 min. A reduction in the PL intensity (~80%) was observed only for CO at a 22

547

concentration of 200 ppm, following a concentration increase from 0 to 200 ppm. The results

548

were fairly reproducible and the process reversible (by heating up to ~90 oC) leading to

549

regeneration of the sensing monolayer device.

550 551 552 553

Figure 15: Schematic of selective detection of carbon monoxide (CO) through complexation strategies developed by Gulino group wherein either a bimetallic Rh complex and photoluminescence transduction technique (A) [39] or a Ru-polypyridyl complex and UV-vis spectroscopy transduction technique (B) [111] were used.

554

Among other systems, porphyrin-based thin films have been increasingly used for gas

555

detection and solid-gas interfacial studies, with either a central metal ion or the peripheral

556

functional groups mediating the processes responsible for the sensing [112, 113]. Utilizing

557

the above strategy, Gulino et al. successfully demonstrated the utility of a porphyrin

558

monolayer on quartz substrate for selective and sensitive detection of even 1 ppm NO2 using

559

an ‘off-the-shelf’ UV-vis spectrophotometer with t1/2 = 4s (Figure 16A) [114]. The basis of

560

the detection was the disappearance of a characteristic Soret band at λmax = 424 nm and

561

appearance of a new band at λmax = 458 nm due to the formation of the porphyrin-NO2

562

adduct. In addition, the effect of humidity on the NO2 detection was studied; the presence of

563

both NO2 and humidity led to overlap of three bands at λ = 424, 448, and 463 nm (t1/2 = 72s)

564

corresponding to the Soret bands of the starting, protonated, and porphyrin-NO2 adduct,

565

respectively. Further, it was demonstrated that introduction of dilute inorganic acid such as

566

HCl leads to protonation of the porphyrin, so that a successive NO2 exposure did not result in

567

any perturbation in the UV-vis spectrum of the porphyrin-HCl adduct. Favorably, heating

568

under vacuum at 80 oC recovered the original state of the porphyrin monolayer. In a further

569

study, Gulino et al. extended the above NO2 sensing methodology using the assembly of Co-

570

porphyrin complexes on fused silica substrates (Figure 16B) [115]. In a similar fashion as

571

above, the presence of NO2 was monitored by UV-vis spectroscopy. Exposure of the 23

572

monolayer to only 10 ppm NO2 in N2 resulted in a red shift (change in wavenumber = 857

573

cm-1) of characteristic optical bands, which confirmed the ability of the system to detect NO2,

574

in good correlation with complementary solution studies.

575 576 577

Figure 16: Representative examples of supported monomolecular sensors for detection of NO 2 which include porphyrin (A, B) and an Os(II)-polypyridyl (C) complexes developed by Gulino et al. (refs 114, 115, and 116, respectively).

578

Within an alternative approach to NO2 detection, Osbp2+ (vide supra) attached to float glass

579

substrates was utilized (Figure 16C) [116] This monolayer was used for reagentless optical

580

detection of NO2 (1-10 ppm) and NOx (800-2550 ppm) by monitoring the characteristic

581

singlet and triplet MLCT bands (λmax = 512 and 692 nm, respectively) in the UV-vis

582

spectrum of the monolayer. The exposure of the monolayer to NO2 in N2 resulted in

583

Os(II)
Os(III) conversion with concomitant disappearance of the MLCT bands and the

584

response time dependent on NO2 concentration. NOx was generated by reacting HNO3 with

585

Cu powder and the resultant brown gas (NOx: 2550 ppm) was exposed to the Osbp2+

586

monolayer. The monolayer was oxidized within ~10s and could be recycled for at least 10

587

times using H2O as regenerator without much degradation as adjudged by UV-vis

588

spectroscopy.

589

Another

590

dodecanoxyphenyl)-20-(p-hydroxyphenyl) porphyrin-based monolayer on quartz substrates.

591

Exposure of the monolayer to O2 in N2 resulted in quenching of the PL signal [117]. The

592

reduction in the PL intensity varied linearly with O2 content for low O2 concentrations (0% <

593

O2 < 2.5%) while a non-linear behavior was observed at the higher concentrations with a

594

detection limit of 0.2%. Favorably, purging the monolayer with N2 stream regenerated the

595

original PL signal. The observed PL quenching was associated with the O2-induced variations

596

in the monolayer structure.

report

from

Gulino

et

al.

described

an

O2

sensitive

5,10,15-tri-(p-

24

597

In another study, Chu et al. developed a highly sensitive, O2-sensing system based on

598

immobilization of fluorescent Ru(II)-polypyridyl complex on a glass substrate using both

599

covalent attachment and Langmuir-Blodgett technique (Figure 17) [118]. In the test

600

experiments, O2 concentration was varied from 0% (100% N2) to 100% with 10% intervals.

601

The increase in O2 concentration resulted in a gradual quenching of the fluorescence intensity

602

which, however, was partly or fully recovered as soon as exposure to O2 was limited or

603

terminated, respectively, with no hysteresis involved. A higher quenching efficiency was

604

achieved in the case of the covalently attached Ru(II) complex, which was attributed to its

605

specific orientation, favorable for the O2 sensing.

606 607 608 609

Figure 17: Schematic of fluorescence quenching upon exposure of a Ru(II)-polypyridyl complex covalently immobilized on glass substrate to O2. The process was fully reversible and well reproducible; the O2 sensing ability was better and easier as compared to other methods where host matrixes such as sol-gel, silica glass, or polymer were employed [118].

610

In another study, Muthukumar et al. designed thin films of meso-tetra(4-pyridyl)porphyrin on

611

glass substrates and monitored their applicability to sensing of HCl gas on the basis of UV-

612

vis spectroscopy transduction technique [119]. Exposure of these films to only 0.04 ppm of

613

HCl gas (with N2 as carrier gas) resulted in enhanced intensity of the Soret band along with a

614

red-shift of this band (change in wavenumber = 2143 cm-1), which was attributed to

615

protonation of the peripheral pyridyl N-atoms. A detection limit of 0.01 ppm and higher

616

sensitivity as compared to solution based sensing studies were achieved. Significantly, an

617

almost complete recovery of the sensor device could be achieved by its exposure to NH3

618

vapors at 10% humidity level, but was less efficient at higher humidity levels.

619

Siloxane-based monolayers are commonly used in microelectromechanical systems as a

620

lubrication layer for they are known to decrease the adhesion at interfaces between different

621

components (stiction reduction) [120]. Using a standard assembly strategy, Xu et al.

622

fabricated a siloxane-based covalently assembled monolayer of N-octyldimethylchlorosilane 25

623

on a glass substrate [121]. Afterward, an ultrathin Pd film was deposited leading to the

624

formation of small Pd nanoclusters which, within the strategy of the lubrication control,

625

helped in reduction of stiction between the palladium and the substrate. At the same time, in

626

context of sensing, this assembly was capable of detecting 2% H2 in just ∼70 ms and was

627

sensitive to even ~25 ppm H2, which was traced by a detectable 2% increase in conductance

628

owing to hydrogen-induced palladium lattice expansion.

629 630

5. Sensors for (bio)molecules:

631

There has always been a mounting need to develop reliable sensors for sensing/distinguishing

632

small molecules of biological significance as well as common biomolecules such as H2O,

633

ascorbic acid, carboxylic acids, cholesterol, and others [79, 122-129]. Molecular sensors

634

prepared on SiOx substrates represent new, potentially powerful tools in this regard.

635

Representative examples of such sensors will be discussed in this section.

636

The presence of moisture in laboratory solvents has always been a cause of serious concern

637

when anhydrous conditions were required. Therefore, quantitative detection of water in these

638

systems is of utmost importance. In this connection, Gupta et al. used Osbp2+ monolayer

639

(vide supra) for detection of H2O in organic solvents, particularly in THF (Figure 18A) [130].

640

At first, Osbp2+ moieties in the sensing monolayer were activated by oxidizing them with

641

redox-active Ce4+ ions, which was accompanied by a reduction in absorbance of a

642

characteristic MLCT band, and then the monolayer was exposed to H2O in THF. According

643

to the experimental data, the Osbp2+ monolayer was capable to sense quantitatively the ppm-

644

level of H2O in THF in the range 10-300 ppm within 5 min exposure time, relying on

645

relatively large changes of optical absorbance associated with the band at λmax = 317 nm. In

646

another study, Niu et al. fabricated a fluorescence-based H2O sensor by covalently

647

immobilizing N-allyl-4-morpholinyl-1,8-naphthalimide (AMN) by photo-copolymerizing

648

with acrylamide, (2-hydroxyethyl)methacrylate and triethylene glycol dimethacrylate on a 3-

649

(Trimethoxysilyl)propyl methacrylate (TSPM) modified glass substrate (Figure 18B) [131].

650

Fluorescence characteristics of this sensor were solvent dependent, therefore, different

651

organic solvents were tested in absence or presence of H2O. It was found that the presence of

652

water in the solvents quenches the fluorescence of the sensor; e.g., an increase of the water

653

content in dioxane from 0% to 70% (v/v) reduced dramatically the fluorescence intensity

654

(λexcitation = 400 nm; λemission = 507 nm) of the sensor as well as induced a red shift in the 26

655

emission band. Significantly, the sensing process was pH independent, reversible, and well

656

reproducible.

657

Among other biologically relevant molecules, dicarboxylic acids have also been targeted

658

using sensor units immobilized on SiOx substrates. These acids are the key components of

659

liquid fertilizers and are also present in water-soluble organic compounds in atmospheric

660

aerosols. Generally, these acids are identified using gas chromatography, which is however

661

quite complicated and time-consuming. In this context, Gao et al. developed a novel approach

662

for selective sensing of dicarboxylic acids (e.g. ethanedioic, malonic, and succinic acids) in

663

contrast to monocarboxylic acids (e.g. formic or acetic acid) by constructing a

664

photophysically active monolayer on a quartz plate substrate. This monolayer had a flexible,

665

long spacer, containing ethylenediamine groups and terminated with a pyrene moiety (Figure

666

18C) [132]. Upon exposure to dicarboxylic acids dissolved in aqueous medium this

667

monolayer exhibited an increase in the intensities of both monomer and excimer emissions at

668

λmax ≈ 380 and 480 nm, respectively. This response was attributed to attachment of

669

dicarboxylic acids to the molecular spacers via hydrogen bonding with the ethylenediamine

670

units. An improvement of the sensor performance, including reduction of the equilibration

671

time for detection of ethanedioic acid from 210 min to 70 min and enhancement in sensitivity

672

from 14 mmol/L to 2 mmol/L, was achieved by the use of the diethylenetriamine-containing

673

spacer as compared to those containing ethylenediamine or 1,3-diaminopropane (Figure 18C)

674

[133]. It was proposed that the lengthening of the spacer led to a better mobility of the

675

receptor moiety and that the presence of more imino groups provided additional H-bonding

676

sites towards dicarboxylic acids. A cumulative effect of these two factors facilitated easier

677

insertion of dicarboxylic acids into the monomolecular film.

678 679 680 681

Figure 18: Schematic of SiOx supported sensors for detection of H2O and dicarboxylic acids. (A) An Os(II)-polypyridyl complex based sensing of water in organic solvents, particularly in THF [130]; (B) N-allyl-4-morpholinyl-1,8-naphthalimide

27

682 683 684 685 686

film based sensor for H2O (the highlighted part emphasizes linkage through the polymer) [131]; (C) molecular sensors for detection of dicarboxylic acids, assembled on quartz substrates. The sensing mechanism was based on quenching of fluorescence upon interaction of the monolayer with dicarboxylic acids. The spacer length and number of hydrogen bonding sites (NH groups; see the highlighted parts of the spacers) played an important role in increasing the efficiency of the sensing process [132, 133].

687

Similar to dicarboxylic acids, the detection of industrially relevant amines like n-

688

propylamine, n-butylamine, n-heptylamine etc. is quite important since they can pose

689

significant danger if their concentration exceeds certain permissible exposure limit. For their

690

detection, Comes et al. developed a zeolite beta based chromogenic sensor system where the

691

relevant dyes, particularly, triphenyl pyrilium (S1) and squaraine dyes (S2), were anchored

692

onto pore walls of zeolite beta through N-methyl,N-(propyl-3-trimethoxysilyl)aniline (Figure

693

19) [134]. Even though these dyes react with a number of amines, the detection process was

694

sluggish in solution. In contrast, the substrate-anchored dyes showed unambiguous

695

chromogenic responses towards amines including small ones like n-propylamine, n-

696

butylamine, and n-heptylamine as well as bulky amines like 9-methylaminoanthracene, 3,3-

697

diphenylpropylamine and others at pH = ~10 and in solvents of different polarity, viz, water

698

and ethanol. In ethanol, less bulky amines could reach the dyes anchored inside the pores and

699

displayed rapid magenta to yellow color changes due to nucleophilic substitution reaction

700

while the bulky amines could not induce any color changes even in weeks. However, in

701

water, smaller and more polar amines tended to form H-bonds with water and hence, only

702

more hydrophobic amines like n-heptyl amine could be detected efficiently.

703 704 705

Figure 19: Pyrilium based dye (S1) and squaraine based dye (S2) immobilized onto aniline terminated pore walls of zeolite beta for chromogenic detection of different amines [134].

706

Small, physiologically relevant molecules like ascorbic acid, dopamine, and uric acid are

707

typical components of extracellular fluids, coexisting with each other. The oxidation

708

potentials of these molecules are similar which poses a serious challenge to their qualitative

709

and quantitative analysis. In this context, we have developed a monolayer-based system for

710

selective detection of ppm levels of ascorbic acid (AA) with and without potent interferents 28

711

[50]. The system relied on the Cu2+/+ redox chemistry and used UV-vis spectroscopy

712

transduction technique for the detection. Two different Cu(II)-terpyridyl complexes were

713

immobilized on glass substrates, terminated with either acetate moiety (A) or chelating 1,10-

714

phenanthroline group (B) (Figure 20). Exposure of the former film to only 5.5 ppm of AA

715

for ~5 min to (A) led to appearance of a MLCT band centered at λmax = 423 nm indicative of

716

reduction of Cu2+ to Cu+ and to a blue shift (change in wavenumber = ~646 cm-1) of another

717

absorption band (λmax = 356 nm), while similar behavior was not observed for (B). The

718

observed hypsochromic shift was ascribed to a change in the intermolecular interaction as a

719

result of the intra-rotational conformational transformation in (A), which was not possible in

720

the case of (B) owing to the chelate and rigid coordination of the phenanthroline group

721

(Figure 20). In addition, it was demonstrated that the sensor can be easily regenerated by

722

keeping it in air/O2 or in O2 saturated NaOAc solution for ~5 min, with the latter procedure

723

resulting in a full regeneration.

724 725 726

Figure 20: Schematic of sensing of AA by Cu(II)-terpyridyl complexes immobilized on glass substrates, where acetate terminated complex could reversibly sense AA while the phenanthroline terminated one could not [50].

727

For selective detection of dopamine, Cejas et al. assembled fluorescent diazapyrenium films

728

on quartz, glass and silica substrates (Figure 21) [135]. When these films were optically

729

excited at 342 nm, they showed characteristic emission at λ = 400-500 nm. Exposure to only

730

sub-millimolar levels of dopamine in an aqueous medium at neutral pH resulted in a

731

noticeable reduction of the emission intensity from the sensor, while the presence of a large

732

excess of ascorbic acid did not affect the sensing process. This behavior was attributed to

733

supramolecular π-π stacking interactions between the electron-rich catechol unit in dopamine

734

and electron-deficient diazapyrenium dications.

29

735 736 737

Figure 21: Immobilization of diazapyrenium dications on SiO x substrates for fluorescence-based selective detection of dopamine [135].

738

Among other physiologically relevant molecules, cholesterol is of particular importance since

739

a high cholesterol level represents a major risk factor for vascular diseases, coronary heart

740

diseases, and other dysfunctions. Therefore, rapid and reliable cholesterol analysis is highly

741

desirable, defining the need for respective biosensors. In this context, Arya et al. developed a

742

cholesterol biosensor by immobilizing cholesterol oxidase onto a SAM of N-(2-aminoethyl)-

743

3-aminopropyl-trimethoxysilane prepared on an ITO-coated glass substrate via N-ethyl-N′-(3-

744

dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide (EDC/NHS) chemistry

745

(Figure 22A) [136]. The authors used electrochemical impedance spectroscopy to

746

discriminate between the hydrolyzed ITO and cholesterol oxidase functionalized ITO; the

747

charge transfer resistance, RCT, for the latter (∼292 ohms) was much higher than that for the

748

former, indicating immobilization of cholesterol oxidase. These cholesterol sensing

749

bioelectrodes showed a linear response towards cholesterol in the range from 50 to 500 mg/dl

750

with a detection limit of ca. 25 mg/dl. High sensitivity at 4.499 × 10 −5 Abs (mg/dl)−1 with a

751

shelf life of ~10 weeks and an electrode reusability of 10 times position this cholesterol

752

biosensor as one of the best reported; its performance is better than that of analogous

753

biosensors employing conducting polymers, sol-gels, carbon electrodes, and different

754

monomolecular films [137-139].

755

One of the most important and commercialized biosensors has been devised for monitoring

756

blood glucose, which is in particular necessary in the case of diabetes. For instance, Gun et al.

757

developed a field-effect-based glucose biosensor by successively modifying the Si/SiO2

758

substrate with 3-mercaptopropyltrimethoxysilane (MPTMOS), gold nanoparticles (Au-NPs),

759

and enzyme glucose oxidase (Figure 22B) [140]. It was also observed that a co-

760

immobilization of ferrocene redox species could result in a two-fold increase of the biosensor

30

761

sensitivity. This effect was explained by the hydrogen peroxide-mediated oxidation of

762

ferrocene resulting in a pool of charged species at the interface, improving the sensor

763

response towards glucose in the constant capacitance mode. The above approach was

764

suggested as a general strategy to amplify signals from field-effect based biosensors.

765 766 767 768

Figure 22: Schematic of biosensors fabricated for (A) sensing of cholesterol using electrochemical impedance spectroscopy [135]; (B) sensing of glucose using constant capacitance mode [140]; and (C) sensing of CML using complementary CML probe (red wavy lines) attached to Cd-Te quantum dots [141].

769

SiOx substrates have also been employed for developing sensors for cancer biomarkers. In

770

particular, Sharma et al. fabricated monolayers of CdTe-QDs (QD = quantum dots), capped

771

with thioglycolic acid, on (3-aminopropyl)-trimethoxysilane functionalized ITO-coated glass

772

substrate using EDC/NHS chemistry (Figure 22C). This biosensor was utilized for detecting

773

markers of chronic myelogenous leukemia (CML; blood cancer) [141]. The specifically

774

designed, synthetic capture probe for these markers, when covalently immobilized on the

775

QDs modified surface, was specifically hybridizing with its complementary DNA in the

776

concentration range from 1 µM to 1 pM within 30 min. The authors claimed that this device

777

can serve as a supplementary tool for initial screening of CML.

778 779

6. Sensors for explosives and warfare agents:

780

The deliberate use of chemical and biological warfare agents (nerve agents, sulfur mustard,

781

anthrax, and others) is a cause of serious concern in view of growing extremism in various

782

parts of the world [53, 122, 142]. Also, explosive materials such as trinitrotoluene (TNT),

783

trinitrotriazine (RDX), and others pose a serious threat [125]. Therefore, there is a growing

784

need to have suitable technological means for detection of such materials to combat

785

underlying threats. In this context, Zhang et al. constructed pyrene-based fluorescent

786

monolayers on glass substrates with different pyrene densities and spacer lengths (Figure

31

787

23A) [143]. These monolayers could be used for vapor phase, ppb level detection of

788

nitroaromatics (NACs), viz., trinitrotoluene (TNT), dinitrotoluene (DNT), and nitrobenzene

789

(NB) (sensitivity order: NB > DNT > TNT), based on quenching of fluorescence, while the

790

potentially interfering molecules like benzene, toluene, ethanol, and trinitrophenol as well as

791

the presence of smoke did not induce such an effect. Favorably, treatment with ethanol and

792

subsequent rinsing with water could regenerate the sensor for potential reuse. Interestingly,

793

the monolayer having a lower density of pyrene and containing diaminopropane subunit in

794

the spacer provided a better sensing performance than the others, since, following this design,

795

the sensing molecules adopted conformation associated with the formation of a perfect

796

excimer.

797

Within another study from the same group, He et al. designed a monomolecular assembly of

798

oligo(diphenylsilane) on a glass plate substrate for sensing of NACs, viz., TNT and DNT in

799

the vapor phase (Figure 23B) [144]. The sensing relied on monitoring of fluorescence from

800

the monolayer (λemission = 410 nm) which was quenched by 85% upon exposure to NACs

801

(TNT) for 30s. The quenching process was static and reversible in nature as the monolayer

802

could be regenerated by washing with ethanol. The extent of the quenching followed a trend:

803

DNT > TNT > NB > PA (picric acid) > PHDH (2,4-dinitrophenyl hydrazine). This behavior

804

was attributed to differences in vapor pressure, redox potentials, and concentration of the

805

analytes on the film surface. HCl and SO2 were slightly interfering with the detection process

806

while chemicals like benzene and ozone as well as the presence of smoke made no effect on

807

the fluorescence intensity. Further, the same group presented another pyrene-based sensor

808

with benzene moiety in the spacer (Figure 23C) [145]. This sensor was able to detect PA

809

selectively in contrast to other NACs, chloroform, HCl, NaOH, ethanol, and seawater. The

810

detection limit approached 10-8 M. The detection was based on quenching of excimer

811

emission of pyrene moieties [145]. The selectivity to PA was attributed to specific binding of

812

this compound to the imino group, mediated by the proton transfer. The introduction of the

813

benzene moiety into the spacer was important as well since it compelled the pyrene groups

814

extending at the film-ambient interface and also restricted their motions. Further, in a recent

815

study by the same group within the similar concept, another pyrene-based fluorescent sensor

816

was developed which showed a strong quenching of the fluorescence signal upon exposure to

817

picric acid (as a component of PA, DNT, and TNT) in aqueous solutions (Figure 23D) [146].

818

The same group has also constructed another DNT sensing monolayer, consisting of

819

arylethenyl-branched s-triazine on n-propylchloride triethoxysilane functionalized glass 32

820

substrates with the molecule plane of the adsorbate being parallel to the surface plane (Figure

821

23E) [147]. This strategy allowed a better control over the π–π interactions between the

822

grafted molecules. Exposure of this monolayer to DNT (~10-5 M in THF) resulted in the

823

quenching of the fluorescence signal without any noticeable time delay. Favorably, this

824

process was reversible, at least up to 10 cycles, as tested experimentally.

825 826 827

Figure 23: Schematic of monomolecular sensors for explosives (TNT, DNT, NB, PA, and others) fabricated on SiOX substrates by Fang group [143-147].

828 829

Applying another tactic for TNT sensing, Chen et al. developed a microcantilever-based

830

piezoresistive Wheatstone bridge combined with a specially designed sensing bilayer on SiO2

831

substrate to detect 0.1 ppb levels of TNT by measuring surface stress before and after the

832

TNT exposure with no signal attenuation even after 140 days [148]. The bilayer was

833

constructed using p-aminobenzoic acid (PABA) and 3-glycidoxypropyltrimethoxylsilane

834

(GPTS) as precursor materials; the sensing experiments were performed with a lab-made

835

testing cell. Acidic terminal groups (−COOH) of the sensing bilayer could bind to the NO2

836

groups of TNT via H-bonding, which led to the generation of a response voltage, measurable

837

by the microcantilever. It was shown that three successive sensing cycles can be performed

838

within ~35 min with as little as ~10 % error. Following this study, the same group recently

839

described another microcantilever-based sensor utilizing double layer silicon-on-insulator

840

technology and surface modified with the sodium salt of carboxyethylsilanetriol [149]. As

841

proposed by the authors, this highly sensitive (ppb levels of TNT), piezoelectric sensor can be

842

potentially used for on-spot detection of TNT.

33

843

An important class of explosives are 1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-

844

1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) used in improvised explosive devices (IEDs). In

845

context of their detection, Gopalakrishnan et al. have recently reported a fluorescence-based

846

sensor for attogram (ag) level of RDX. The sensor utilized a conjugated polymer film

847

modified with trisphenylene vinyl (TPV) and deposited on allyl silane functionalized fused

848

SiO2 substrate (Figure 24) [150]. Earlier, the same group reported that exposure to RDX can

849

quench the fluorescence of TPV but faced several procedural problems [151] which,

850

however, were later overcome by the immobilization of the TPV modified polymer [150].

851

Note that, according to the authors, the high sensitivity of the device to RDX could only be

852

achieved under a precise control of the polymer film thickness as well as the extent of

853

conjugation in the polymer matrix (in terms of stilbene content in the polymer network

854

measured as absorption ratio at λmax = 280 and 350 nm).

855 856 857

Figure 24: Schematic of 1,3,5-trinitro-1,3,5-triazine (RDX) sensor developed by Gopalakrishnan et al. by immobilizing trisphenylene vinyl modified conjugated polymer on SiO x substrate [151].

858

In context of biohazard control, Yilmaz et al. developed a novel monolayer-based sensor to

859

detect nanomolar levels of anthrax biomarker, dipicolinic acid (DPA) [152]. Within the

860

respective design, a monolayer of β-cyclodextrin was assembled on a glass substrate and

861

subsequently modified using adamantane appended EDTA and naphthalene (antenna) based

862

moieties (called molecular printboards), which resulted in a blue emission in the fluorescence

863

spectrum of the resulting assembly. In the final step, complexation of these moieties with

864

Eu3+ was performed resulting in a red emission in the fluorescence spectrum. Thereafter, the

865

sensors were exposed to aqueous solutions of DPA with different concentrations at pH 6.5 for

866

~10 min and with continuous stirring. This resulted in reappearance of blue emission, which

867

was ascribed to the displacement of the naphthalene moiety by DPA, accompanied by

868

termination of the associated energy transfer between the antenna and the Eu 3+ center. This

869

conclusion was further supported by independent UV-vis spectroscopy experiments. DPA 34

870

detection limit was estimated at 25 nM and the detection efficiency was not affected by the

871

addition of 200 nM of other aromatic ligands such as the o/m/p-phthalic acids, nicotinic acid

872

and its two isomers (picolinic and isonicotinic acids) as well as nicotinamide adenine

873

dinucleotide (NAD).

874

As an example for sensing of a chemical warfare, our group developed a monolayer-based

875

approach to detect sulfur mustard analog, 2-chloroethyl ethyl sulfide (CEES) [53]. In the

876

respective study, a Mg-porphyrazine based complex was immobilized on glass/silicon

877

substrates (Figure 25) and fully characterized by XPS, with the spectra exhibiting a

878

characteristic 2p emission of oxidized Mg at ~51.0 eV as well as characteristic emissions

879

corresponding to different nitrogen atoms. It was also observed that the Q-band in the UV-vis

880

spectrum of the monolayer had both bathochromic and hypsochromic shifts upon exposure to

881

only ~7 ppm of CEES (in CHCl3/CH3OH mixture). Only ~40 ppm of CEES was enough to

882

saturate the sensor, along with sufficient selectivity in the presence of potent interferences

883

like CO2, NOx, water vapor, and smoke.

884 885 886 887

Figure 25: Schematic of immobilization of Mg-octapyridyl porphyrazine complex on a glass substrate. The immobilized complex was used for the detection of sulfur mustard analogue (chloroethyl ethyl sulfide) via UV-vis absorption spectroscopy [53].

888

In a study by Tudisco et al., cavitand functionalized porous silicon substrates (PSi) were

889

employed for detection of dimethyl methylphoshphonate (DMMP), a potentially toxic and a

890

chemical warfare agent similar to sarin (a phosphorous-based nerve agent) [153]. Hydrogen

891

terminated, porous silicon substrates having a large surface area were functionalized with

892

cavitands containing four undecylenic-units via hydrosilylation reaction. Two isomeric

893

cavitands bearing –COOH groups at the upper rim (PSI-Ac-IN and PSi-Ac-OUT) and one 35

894

without any –COOH group (PSi-MeCav) (Figure 26) were exposed to DMMP vapors (0.3

895

Torr) and the response was monitored by XPS and FT-IR spectroscopy. The same group also

896

performed thermal decomposition of DMMP-cavitand-PSi complex (1) and monitored the

897

desorbed product using mass spectroscopy. In the case of the –COOH functionalized

898

cavitands, the attachment/interaction of DMMP to/with the monolayer (via formation of

899

hydrogen bonds) was evidenced by appearance of characteristic doublet at 1035 and 1058

900

cm-1 in the FT-IR spectrum as well as by the presence of characteristic P 2p emission at 134.8

901

eV in the XPS spectra. These features were not observed for the cavitands where –COOH

902

group was absent. Favorably, heating (1) up to 60 oC or rinsing it with N2 for 15 min resulted

903

in decomplexation, allowing reuse of the device. Further, theoretical modelling suggested that

904

these cavitands-based receptors could be ideal candidates for detection of such dangerous

905

nerve gases like sarin.

906 907 908 909 910

Figure 26: Schematic of isomeric cavitands containing –COOH groups (A and B) and a cavitand without –COOH group (C) immobilized on porous silicon substrates for the detection of DMMP. H-bonding interactions between DMMP and the −COOH groups were the driving force for their coordination. These cavitands, in theory, can also detect sarin, a highly potent chemical warfare agent [153].

911

In another study, devoted to detection of nerve agent mimics, Climent et al. produced silica

912

nanoparticles functionalized with 3-mercaptopropyltrimethoxysilane (–SH termination) and

913

3-[bis-(2-hydroxyethyl)amino]propyltriethoxysilane (–OH termination) (Figure 27) [154].

914

These nanoparticles, suspended in CH3 CN, were then exposed to several nerve agent mimics

915

having P−X linkage (X = F, Cl, CN, etc.) viz., DFP (diisopropyl fluorophosphate), DCP

916

(diethylchlorophosphate),

917

chlorosulfidophosphate) followed by subsequent addition of a squaraine dye in CH3CN.

918

Significantly, the squaraine dye could react with the –SH groups resulting in bleaching of

919

characteristic blue colour of the dye. The efficiency of this reaction was however affected by

920

the presence of the nerve agent mimics which reacted preferentially with the –OH groups,

921

thereby forming a dense organophosphate layer hindering the reaction of squaraine dye with

DCNP

(diethylcyanophosphonate),

and

DCSP

(dimethyl

36

922

the –SH groups. Therefore, the bleaching of dye did not occur in the presence of these

923

mimics. The process was monitored by UV-vis spectroscopy where decrease in absorbance of

924

dye at λmax = 673 nm signified the bleaching of the dye. The strategy was suitable for

925

selective detection of DFP even in presence of water and other nerve agent mimics with

926

detection limit approaching ~2 ppt (in 5-10% water).

927 928 929 930

Figure 27: Schematic illustration of the surface of the di-functionalized silica nanoparticles utilized for detection of nerve agent mimics by inhibition of transport of squaraine dye towards the nucleophilic –SH terminal groups. The –OH groups could get easily phosphorylated than the bulky –SH groups [154].

931 932

7. Conclusion and perspective

933

Controlled assembly on solid substrates provides a means to create highly ordered,

934

specifically oriented, and densely packed arrangements of functional molecules which have

935

significant advantages as compared to the respective disordered systems, such as these

936

molecules in solution. These advantages make supported molecular films a strong candidate

937

for diverse applications, including sensor fabrication. A particular important support in

938

context of surface-confined molecular sensors are SiOx substrates which have certain

939

advantages over metal substrates, primarily due to the covalent character of the molecular

940

attachment, providing robustness to the assembled films and permitting their subsequent

941

modification and/or functionalization without their deterioration. Additionally, SiOx

942

substrates allow usage of optical, electrochemical, mass sensitive and other transduction

943

techniques to monitor molecular recognition events.

37

944

In this review, we focused on most recent and representative studies in which custom-

945

designed molecular films on SiOx substrates were utilized for sensing of cations/anions,

946

gases, biomolecules, as well as chemical and biological warfare agents. Particular aspects

947

addressed in this review are the versatility of SiOx substrates, surface modification

948

procedures, diversity of molecular films and their subsequent modifications as well as

949

different transduction techniques. The relevant processes upon the sensing events included

950

electron-transfer at solution-surface interface, changes in the optical spectra, quenching or

951

enhancement of fluorescence, redox changes, and others, which could be monitored by such

952

techniques like UV-vis spectroscopy, fluorescence spectroscopy and microscopy, FT-IR

953

spectroscopy, XPS, AFM, electrochemical tools, and several others. However, there is still a

954

lot of work to be done, in particular with regard to anion sensing which is an especially

955

difficult task but of utmost significance [155]. Formation of surface-confined, interlocked

956

structures where anions act as templating agents or the concept of molecular machines could

957

be possibly useful in this context [156, 157]. Also, combination of different kinds of

958

receptors, viz., coordination compounds, low molecular mass fluorophores, conjugated

959

polymers etc. could be a useful strategy to obtain new sensors with multiple functionalities

960

and/or superior performances, where these receptors can function independently or

961

synergistically. Further, suitably functionalized surface-attached metal- or covalent organic

962

frameworks (SURMOFs or SURCOFs) can be other potential candidates for highly sensitive

963

and selective detection of different analytes [158].

964 965

Acknowledgements

966

VS and AKS gratefully acknowledge the support of Department of Science and Technology

967

INSPIRE grant (DST/INSPIRE/04/2015/002620) and (DST/INSPIRE/04/2015/002555),

968

respectively. PCM thanks National Research Council of Canada and Alberta Innovates

969

Technology Futures. MZ acknowledges financial support of the German Research

970

Foundation (DFG).

971 972

References

973 974 975 976 977

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HIGHLIGHTS

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Advantages of SiOx-based substrates are discussed. Design and fabrication of molecular sensors on SiOX substrates are briefly reviewed. Examples including sensing of ions, small molecules, biomolecules, warfare agents are reviewed. Focus is on detailing the sensor attachment chemistry and relevant sensing chemistry and their advantages over solution analogues.

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