Oxinate-Aluminum(III) Nanostructure Assemblies Formed via In-situ and Ex-situ Oxination of Gold-Self-Assembled Monolayers Characterized by Electrochemical, Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy, and X-ray Photoelectron Spectroscopy Methods

Accepted Manuscript Title: Oxinate-Aluminum(III) Nanostructure Assemblies Formed via In-situ and Ex-situ Oxination of Gold-Self-Assembled Monolayers Characterized by Electrochemical, Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy, and X-ray Photoelectron Spectroscopy Methods Author: Reza Karimi Shervedani Zeinab Rezvaninia Hassan Sabzyan PII: DOI: Reference:

S0013-4686(15)30227-9 http://dx.doi.org/doi:10.1016/j.electacta.2015.07.166 EA 25448

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

18-2-2015 28-7-2015 28-7-2015

Please cite this article as: Reza Karimi Shervedani, Zeinab Rezvaninia, Hassan Sabzyan, Oxinate-Aluminum(III) Nanostructure Assemblies Formed via In-situ and Ex-situ Oxination of Gold-Self-Assembled Monolayers Characterized by Electrochemical, Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy, and X-ray Photoelectron Spectroscopy Methods, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.07.166 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Oxinate-Aluminum(III) Nanostructure Assemblies Formed via In-situ and Ex-situ Oxination of Gold-Self-Assembled Monolayers Characterized by Electrochemical, Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy, and X-ray Photoelectron Spectroscopy Methods

Reza Karimi Shervedani, * Zeinab Rezvaninia, Hassan Sabzyan Department of Chemistry, University of Isfahan, Isfahan 81746-73441, I. R. Iran. *

To whom correspondence should be addressed. Tel.: +98-31-37934922. Fax: +98-31-36689732. E-mail address: [email protected] (R. K. Shervedani).

Graphical Abstracts-MS ST15-075

EIS complex plane plots A: In-situ approach: (a) Bare Au, (b) Au-MPA, (c) Au-MPA-5A8HQ, and (d) Au-MPA5A8HQ-Al(III) B: Ex-situ approach: (a) Bare Au, (b) Au-MPA, and (c) ex-situ prepared Au-MPA5A8HQ:Al(III)

1

Research Highlights-MS ST15-075  Assembling new oxinate-Al(III) nanostructures on gold surface via in-situ and ex-situ methods.  Structures are characterized by CV, DPV and EIS, as well as by ATR-FTIRS and XPS methods.  Results show successful construction of nanostructures via both methods, but with different features.  Pure & ordered

nanostructures

were

obtained

Au-MPA-oxinate-Al(III).

2

via

ex-situ

formation

of

Abstract Oxinate-aluminum nanostructures constructed on gold-mercaptopropionic acid using 5-Amino-8-hydroxyquinoline complex of aluminum(III), Au-MPA-5A8HQ-Al(III), are prepared for the first time via in-situ and ex-situ approaches, and their physiochemical characteristics are studied by cyclic and differential pulse voltammetry, electrochemical impedance spectroscopy, attenuated total reflectance Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The electrochemical signal background observed for the in-situ prepared nanostructures is large and superimposed by some faradaic effects, while, it is small, smooth, and featureless for the ex-situ prepared nanostructures. To find the source of these features, effects of several parameters (like solvent, preparation method, and linking spacer between the complex and Au surface) are studied. These features are attributed to the intercalation and physical adsorption of the

free 5A8HQ

molecules onto

the

Au-MPA-5A8HQ structure when prepared via in-situ method. Nanostructures with minimum backgrounds in their electrochemical responses could be obtained via in-situ assembling of 5A8HQ from DMF solvent onto the Au-MPA surface, and then, accumulation of Al(III) onto the 5A8HQ layer, and also via ex-situ formation of 5A8HQ:Al(III) complex in the ethanol phase first, and then, transferring the complex onto the Au-MPA surface. The equilibrium constants for the intercalation and adsorption processes are calculated for the first time based on the collected experimental surface quantities.

Keywords: Oxinate-Al(III)

Nanostructures;

Surface

Functionalization;

Electrochemical Impedance Spectroscopy; ATR-FTIR, XPS.

3

Voltammetry;

1. Introduction 8-Hydroxyquinoline (8HQ) can be considered as a bidentate ligand containing two coordinating sites; one the pyridine donor of the bipyridine section and the other the phenolate unit of catecholate section [1]. The 8HQ unit

is an ideal building block in

metallosupramolecular chemistry. Thus, the 8HQ and its derivatives have been immobilized on different surfaces including polystyrene derivatives [2,3], resins and zeolites [4-6], cellulose [7], polyacrylonitrile [8], malachite [9], modified glass [10], silica [11-13] and silver [14]. The 8HQ is a bifunctional hydrogen bonding (H-bond) molecule, which acts simultaneously as a H-donor via the O-H group and as an H-acceptor at the N-atom in aqueous or alcoholic solutions. Because of short distance between the OH group and the N atom (aza group) of the neighboring ring, the OH and aza groups may contribute to a keto-enol tautomeric equilibrium [15]. Accordingly, the inter- and intra-molecular H-bond allow different toutomers/conformers of 8HQ in different environments (solvents); i.e. cis-8HQ with intra-molecular H-bond, and trans-8HQ without intra-molecular H-bond, depending on the nature of the solvent. Inter-molecular H-bond interaction (in trans-8HQ) is expected to be dominant in solvents like water, while intra-molecular H-bond interaction (in cis-8HQ), is the main effect in solvents like DMF and DMSO [16,17]. This behavior is considered to describe the electrochemical response background features observed for the gold electrode surface modified by in-situ approach in the present work. The 8HQ group can form an interesting complex with aluminum ion known as tris-(8-hydroxyquinolinato)aluminum (AlQ3) [18,19]. Due to its effective electron transport properties as well as its high luminescence efficiency, the AlQ3 has been extensively studied for organic light emitting diodes (OLEDs) [20-22], electronic applications as thin films [19], and electrode modifying films for solar cells [23].

4

Recently, the AlQ3 complex has been fabricated on different solid surfaces like mesoporous silica [24,25], polymers [26], and alumina (Al2 O3) [22] to increase their processability. The materials and thin films modified with AlQ3 moieties accompanied by Au or Au nanoparticles are especially important for applications in electronic [27,28] and optoelectronic devices [18-22]. In addition, synthesis of the complexes of amino-substituted 8HQ with Al(III) as promising charge transport buffer layer between a photoactive layer and the gold anode in the organic solar cells have been reported by Manninen et al. [23]. However, to our knowledge, oxination of gold electrode surface and its interaction with Al(III) by using 8HQ or its derivatives via thiol self-assembled monolayers (SAMs) have not been reported yet. Stable, reproducible and compact thiol SAMs can be easily synthesized on the metal surfaces especially on the gold surface [29,30]. The structure of SAMs can be controlled at molecular scale for engineering and designing the electronic devices [31,32], therefore, SAMs are themselves nanostructures [33]. These nanostructures have served as interesting platforms to address many physicochemical questions about electron transfer process at the electroactive [34,35] or electroinactive [36-38] moieties by confining the process at the electrode/solution interface. It is also well known that SAMs can increase or decrease electrode work function depending on the polarization of the dipole layer [39-42]. Therefore, SAMs can be used to control the work function of a metal in photonic devices. A short review on the importance of the gold-thiol-AlQ3 SAMs in photonic devices, advantages and also disadvantages of gold-based substrates in photonic devices and luminescence properties of them are given at the beginning of the Supplementary Data. Gold-mecrcaptopropionic acid (Au-MPA) SAMs have usually used as initial stages for many gold functionalization. The carboxylate functions of Au-MPA SAMs activated by 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide

5

hydrochloride/N-hydroxysuccinimide

(EDC/NHS) organic activators are appropriate places to form covalent bond with amine groups of the desired functional structures [43]. In our previous work [38], analytical application of one type of these structures, functionalized with 8HQ for recognition and quantitative measurement of Al(III), i.e. the oxinate-Al(III) structure, was intended. Accordingly, we constructed a new sensor and explained its analytical aspects for Al(III) measurement. To improve the merits of the method some parameters like dynamic response range, detection limit, sensitivity, repeatability of the sensor response, and especially reversible adsorption of Al(III) ions were considered, and the sensor was designed, with no stress on the characterization of the prepared structure of the sensor at molecular level. The

present

work

oxinate-aluminum(III)

is

intended

to

form

nanostructures

stable,

reproducible,

and

irreversible

on

the

Au-MPA

surface

using 5-Amino-8-hydroxyquinoline (5A8HQ). Two routes, namely in-situ and ex-situ methods, are tested to prepare these nanostructures stressing on their high purity and ordered assemblies. To characterize the modified surfaces, electrochemical methods, including cyclic and differential pulse voltammetry (CV and DPV), electrochemical impedance spectroscopy (EIS), attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) techniques are used. The ATR-FTIR spectroscopy is a sensitive technique to the coordination environment around the metal ions [44-46]. Details of the immobilization process and its optimization and characterization are described in the next section. One of the main advantages of the ATR-FTIR technique is the sampling of a thin (submicron) layer of the surface, thus allowing analysis of the thin layers; especially it is appropriate for studying the SAM immobilized on metallic surfaces. The metallic surfaces have no contribution to the ATR signal, therefore, the

6

ATR-FTIR spectroscopy can be used to collect vibrational data from the SAM immobilized on the metallic surfaces [47]. Alternatively, the XPS measurements allow assessing the presence, the stoichiometry and oxidation states of the elements in the film components [48,49].

2. Experimental 2.1. Materials and reagents Al(NO3)3.9H2O (5A8HQ),

(99.999%),

8-hydroxyquinoline

(8HQ),

1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide

5-amino-8-hydroxyquinoline hydrochloride

(EDC),

N-hydroxysuccinimide (NHS), 3-mercaptopropionic acid (MPA), glycine (Gly), 4-amino thiophenol

(4-ATP),

parabenzoquinone

(PBQ),

ferrocenemethanol

(FcMeOH),

hexacyanoferrate ([Fe(CN)6]3−/4-), DMF (99.5%), with analytical grade, chloroform (CHCl3, 99.4%), ethanol (99.5%) and other chemicals were obtained from commercial sources (Merck® or Sigma®) and used as supplied, except PBQ which was purified by the method explained elsewhere [50]. Phosphate buffer and acetate buffer saline (PBS and ABS) solutions were

prepared

at

desired

pHs

using

0.1 M

NaH2PO4/Na2HPO4

or

0.05M

CH3CO2HCH3CO2Na, respectively. Supporting electrolyte for buffers was 0.1 M NaClO4 for all electrochemical measurements. The working solutions of Al(III) were prepared in ABS solution at pH 5.0 from 0.10 M Al(III) stock solution. The Al(III) stock solution was prepared by dissolving required amount of Al(NO3)3.9H2O in 0.01 M nitric acid. To avoid contamination, the vessels were soaked in 3 M HNO3 and carefully cleaned before use.

2.2. Preparation of the Au-MPA-5A8HQ-Al(III) SAM electrode Polycrystalline gold disk working electrode (Au 99.99%, 0.0707 cm2 surface area, Azar electrode Co., Urmia, I.R. Iran) was cleaned according to our previous reports [34-36,51]. The

7

cleaned Au electrode was immediately immersed into 20.0 mM MPA in aqueous solution at room temperature, sonicated for 10 min, and then, left further in the same MPA solution for 20 h to form the Au-MPA SAM electrode. Then, the electrode was washed with double distilled water and used for functionalization with 5A8HQ via (i) in-situ and (ii) ex-situ approaches as described below. (i) In-situ approach: A set of the Au-MPA SAM electrode was activated in 0.05 M ABS, pH 5.5, containing 4 mM EDC and 5 mM EDC for 1.5 h, and then, rinsed with the same ABS solution and immediately placed into the functionalization solution containing 2 mM 5A8HQ aqueous solution for 2 h to fabricate the Au-MPA-5A8HQ SAM electrode. Then, the electrode was removed from the 5A8HQ solution and rinsed thoroughly with double distilled water to eliminate physically adsorbed species. Finally, the Au-MPA-5A8HQ SAM electrode was immersed in an Al(III) solution at pH 5.0 to form the Au-MPA-5A8HQ-Al(III) SAM electrode. Several solvents were tested in this step, like DMF, CH3Cl and their mixture, acetonitrile, tetrahydrofuran and ethanol, among them DMF was found to be a more appropriate solvent for dissolution of 5A8HQ. The surface modified in this way is represented as Au-MPA-5A8HQ(DMF). When other solvents are tested, they are represented similarly. (ii) Ex-situ approach: Another set of the aforementioned activated Au-MPA electrode was immersed in 2 mM ethanolic solution of 5A8HQ:Al(III) complex for 2 h to form Au-MPA-5A8HQ:Al(III) SAM electrode. The 5A8HQ:Al(III) complex was prepared by mixing a 1:3 ratio of equimolar ethanolic solutions of 5A8HQ and Al(III) precursors, after testing different ratios of 5A8HQ:Al(III) [18]. The electrode was then washed carefully with ethanol solvent and used for electrochemical and spectroscopic measurements. Hereafter, for simplicity, the structure obtained via in-situ and ex-situ approaches are denoted as Au-MPA-5A8HQ-Al(III) and Au-MPA-5A8HQ:Al(III) SAM, respectively. The synthesis routes and the expected structures are presented in Scheme 1.

8

2.3. Instruments and Apparatus Electrochemical experimental setup, including instrumentation, measurements, acquisition and analysis of data, is described in our previous reports [30,34,35]. Briefly, all electrochemical

measurements

were

performed

by

using

Potentiostat/Galvanostat

AUTOLAB 30 (Eco Chemie) in conjunction with Frequency Response Analyzer (FRA), interfaced with a personal computer and controlled by General Purpose Electrochemical System (GPES 4.9) and FRA 4.9 software. The measurements were performed in a three-electrode glass cell (6.1415.150, Metrohm) including the modified polycrystalline gold disk as working electrode, a large area Pt rod as auxiliary electrode (~70 times larger than that of working electrode), and a Ag/AgCl (3 M KCl) electrode as reference. Analysis of the EIS data was performed according to our previous reports [30,34]. The electrochemical cell was placed in a Faraday cage to eliminate any environmental stray field effect. An ultrasonic bath (Bandlin, HF 35 kHz) was used for sonication of the electrodes. All experiments were carried out at room temperature. The ATR-FTIR spectroscopy is used to collect vibrational data from the SAMs immobilized on the surface. The Fourier transform infrared spectrometer (Tensor27, (Bruker), equipped with OPUS software, was used for these measurements. The standard sample cell was a Pike Miracle single-bounce ATR cell equipped with a CsI crystal. Maximum resolution of this instrument and cell is 1 cm1. The ATR spectra of the samples are all referenced to the signal collected from the empty CsI crystal. This procedure eliminates all the peaks corresponding to the contents of the air in the sample compartment, mainly due to the water and CO2. The traveling length of the IR radiation inside the SAM depends on the incident angle (which is 45° here) and the thickness of the SAM (which is less than 2 nm for our samples here). The penetration depth is proportional to the wave vector k = 2π/λ [45].

9

This set-up results in ~106 number of radiation passages through the sample length of 2 nm, producing a spectrum intensity comparable to those collected for macroscopic samples with thickness of ~ 1.0 mm. Electrodes were kept in a vacuumed desiccator for 15 min prior to record the ATR-FTIR spectra. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a VG Microtech Twin anode XR3E2 X-ray source and a concentric hemispherical analyzer operated at a base pressure of 5×1010 mbar using Al K (1486.6 eV). The XPS results in conjunction with cathodic voltammetry desorption of MPA from Au-MPA, were used to assess the stoichiometry of the elements of the film components and their oxidation states.

3. Results and discussion 3.1. Formation of Au-MPA-5A8HQ-Al(III) Modification of the electrode surface leads usually to changes in the properties of the electrode/solution interface, like variation in charge transfer resistance (Rct), double layer capacitance (Cdl), and the charge-state of the surface. The magnitude and direction of these changes depend on the nature of the modifying layer functional groups and the testing solution conditions. Acidity strength of the surface functional groups and the solution pH (charge state of the surface), the nature (kinetic and charge) of the probe molecule, ionic strength () of the solution, and electroactivity of the surface functional groups can be mentioned as examples of the involving parameters [30,34-36,52]. Variations in the properties of the electrode/solution interface can be traced by using an appropriate redox probe dissolved in the solution phase [30,36] or confined on the surface [34,35]. Background currents can also be used for this purpose [53]. The modification steps are traced using faradaic currents of the several external redox probes, and the electrochemical backgrounds and the faradaic-features superimposed on them observed in the absence of any redox probe.

10

3.2. Step-by-step characterization of the Au-MPA-5A8HQ-Al(III) surface 3.2.1. In the presence of external redox probes There is no ideal probe for electrochemical study of the electrode/solution interface; each type of probe can reveal some of the related characteristics. The charged or uncharged redox probe like [Fe(CN)6]3− or FcMeOH, alone cannot help discriminating the parameters affecting electrochemical behavior of the electrode/solution interface including electrostatic interaction, hydrogen bonding, chain length and activity of the adsorbed molecules. These parameters influence significantly on the rate of charge transfer process, magnitude of double layer and surface roughness of the electrodes. The results obtained using charged and neutral probes, as well as the results obtained in the absence of any external redox probe, are essential for a comprehensive assessment of the behavior of the electrode/solution interface. Among different external redox probes [Ru(NH3)6]3+/2+, [Fe(CN)6]3−/4−, PBQ and FcMeOH, tested for this purpose, the charged [Fe(CN)6]3−/4− and uncharged FcMeOH probes were found to be more appropriate to trace the modifications of electrode/solution interface in the present work.

3.2.1.1. Negatively charged probe, [Fe(CN)6]3−/4− The CVs (A), DPVs (B) and EIS (C) data obtained on the clean Au (a), Au-MPA (b), Au-MPA-5A8HQ (c), and Au-MPA-5A8HQ-Al(III) (d) electrodes in the presence of the [Fe(CN)6]3−/4 redox probe are presented in Fig. 1. The probe exhibits a reversible electrochemical behavior at the bare Au and varies upon each step of the surface modification. The EIS data are approximated using the most appropriate model (Fig. 1C, Inset) and numerical methods [54,55], and the values of the model parameters, including Cdl and Rct are extracted and reported in Table 1.

11

The variations in the CV, DPV and EIS signals observed due to the modifications of the surface may be originated from variations of the charge state of the surface, thickness of the assembled film, H-bonds, and redox activity of the film. The resultant of these effects determines the magnitude and direction of the observed variations (Table 1). Formation of the MPA SAM layer on the Au electrode imposes an insulating effect and results in a decrease in the voltammetric peak current (Ip) and an increase in Rct (Ip∝1/Rct). Note that the charge state of the surface in these conditions (pH 5.5) is almost neutral, allowing formation of H-bond between neighboring MPA groups. Alternatively, assembling of the 5A8HQ on the Au-MPA surface causes an increase in Ip and a decrease in Rct. This increase continues further by accumulation (covalent coordination) of Al(III) on the Au-MPA-5A8HQ SAMs. The presence of 5A8HQ increases the film thickness which is clear, and imparts a positive charge to the surface (surface pKa,surface [30,34,52] of the Au-MPA-5A8HQ is more than 6.0 (Supplementary Data, Fig. S1, Insets), causes an attraction between surface and [Fe(CN)6]3/4. Also, the redox activities of free 5A8HQ, which is responsible for the faradaic background currents, and the H-bonds between the assembled 5A8HQ molecules, which cause an insulating effect, are other possible sources contributing to the changes in the electrochemical behavior of the surface. Accumulation of Al(III) ions onto the Au-MPA-5A8HQ surface imparts further positive charges onto the surface (promoting effect) and simultaneously tailors [30,36] the 8HQ functional groups, resultanting causes an increase in Ip and a decrease in Rct. The backgrounds observed at the Au-MPA-5A8HQ electrode are large (Fig. 1A, and Table 1, compare Cdl for b and c cases), which will be discussed in detail separately in Section 3.3.

12

3.2.1.2. Uncharged (neutral) probe, FcMeOH Among several neutral redox probes, like FcMeOH [34,56], ascorbic acid, dopamine [52,57] and PBQ [58] used to trace modification of the electrode surface, the FcMeOH probe was found to be the most appropriate, because it neither modifies the background current significantly nor contaminates the electrode. The CVs (A), DPVs (B), and EISs (C) obtained on

the

clean

bare

Au (a),

Au-MPA (b),

Au-MPA-5A8HQ (c),

and

Au-MPA-5A8HQ-Al(III) (d) electrodes in the presence of 0.5 mM FcMeOH are presented in Fig. 2. The electrochemical parameters extracted from the EIS data are reported in Table 2. The FcMeOH redox probe has exhibited a reversible behavior at the bare Au surface. However, modification of Au by MPA, 5A8HQ, and Al(III) have caused a decrease in the Ips (Fig. 2A and B), and an increase in Rct (Fig. 2C). This is a usual behavior, because the FcMeOH is an uncharged probe and has no significant electrostatic interaction with the surface, and thus, effective insulation (coming from an increase in the thickness and formation of H-bonds) is the dominant effect in determining the magnitude of changes in Ip and Rct here (Table 2). These observations supports successful modification of gold surface, and thus, formation of the Au-MPA-5A8HQ-Al(III) nanostructure assemblies. It should be mentioned here that the background currents obtained on Au-MPA-5A8HQ (Fig. 2A) are large and comparable with those observed in the presence of [Fe(CN)6]3/4 (Fig. 1A). Similar results were also obtained in the presence of [Ru(NH3)6]3+/2+ (the data not reported here). Since these backgrounds are observed in the presence of both neutral and charged probes, they might be originated from the processes related to the modifying film. 3.2.2. In the absence of any redox probe To find the source of the backgrounds, step-by-step modifications of the electrodes surface have been traced in the absence of any redox probe. The results (Fig. 3A) show that the backgrounds obtained on Au-MPA-5A8HQ surface (curve c) are large (compared to those

13

obtained on Au and Au-MPA surfaces, curves a and b) and include some faradaic features, which both decrease after accumulation of Al(III) on the surface (curve d). A decrease in the background currents would be expected after each step of modification if an uncharged and non-electroactive layer is formed on the electrode surface [29,34,35,59]. Therefore, it is necessary to investigate possibility of the presence of electroactive species on the surface. The amine (NH2) group of the 5A8HQ molecule of the Au-MPA-5A8HQ layer is covalently bonded to the Au-MPA, and is not available to undergo a redox reaction [34,50]. Furthermore, the dangling 8HQ group is not electrochemically active in the scanned potential window [60] (Supplementary Data, Figs. S2A and S2B). Therefore, the Au-MPA-5A8HQ electrode is not expected to be electroactive. As a possibility, intercalation and physical adsorption (via H-bond) of 5A8HQ from the modifying solution via its N and OH groups with amide group (CONH) of the Au-MPA-5A8HQ layer, also with the N= and OH groups of the dangling 8HQ moieties (Scheme 1) [61,62] can be considered to be responsible for the large backgrounds observed for this surface (Fig. 3A). Due to the lack of data in literature regarding redox reaction of 5A8HQ and 5A8HQ-Al(III) in their dissolved form in the solution phase, a set of experiments is carried out to examine this possibility (Supplementary Data, Section 2). The results of these experiments support that these compounds are electrochemically active in their dissolved form in the solution phase. One should note that there is no 5A8HQ or 5A8HQ:Al(III) in the solution phase in the conditions of Fig. 3; therefore, physically adsorbed and/or intercalated 5A8HQ molecules are responsible for the observed large backgrounds and their small faradaic features. The amount of adsorbed 5A8HQ can be easily determined by the EIS. The EIS complex plane plots (Fig. 3B), obtained in the absence of any redox probe, are approximated using appropriate equivalent circuit models (Fig. 3B, Insets), and the model

14

parameters Rs, Rct, Cφ, Cdl, g and kapp, are extracted and reported in Table 3. The C, indicating the faradaic features originated from confined redox reaction, is not significant for Au and Au-MPA surfaces, while, it is large for the Au-MPA-5A8HQ electrode. The Cφ value can be used to estimate number of the active sites (Γ = 4RTCφ/n2F2A) involving in the surface confined redox reactions and responsible for the heterogeneous charge transfer [63]. If we assume that Cφ, and thus Γ, is related to the confined redox reaction of 5A8HQ physically adsorbed onto and/or intercalated within the SAMs, then quantitative value of Γ can be obtained in terms of the number of moles of 5A8HQ per unit area (cm2) [34,63]. A value of Γ = 17.39  10−12 mol 5A8HQ cm2 is obtained for the Au-MPA-5A8HQ electrode (Table 3). Similar analysis is performed on the data of the Au-MPA-5A8HQ-Al(III) electrode (Table 3). The results show that quantities related to the nonfaradaic and faradaic background currents (Cdl & C) are both decreased effectively from (1.77 & 4.62) for Au-MPA-5A8HQ electrode to (0.75 & 0.59) for Au-MPA-5A8HQ-Al(III) electrode by factors of (~2.4 & ~7.8). These large decreases observed in the Cdl and C values may be attributed to the displacement of physically adsorbed 5A8HQ molecules from the Au-MPA-5A8HQ surface to the Al(III) ions and simultaneous formation of the Au-MPA-5A8HQ-Al(III)-(5A8HQ)x complex (this possibility is supported by experimental data, Section 3.3.5), or transfer of physically adsorbed 5A8HQ molecules into the solution phase and simultaneous complexation of Al(III) by the 5A8HQ molecules chemically attached to the functional groups of the Au-MPA surface. Large increase observed in Rct upon Al(III) accumulation onto the surface (Rct = 0.11 and 0.77 M for Au-MPA-5A8HQ and Au-MPA-5A8HQ-Al(III), respectively) indicates that tailoring of the Au-MPA-5A8HQ by the Al(III) ion has a significant contribution to the passivation of the Au-MPA-5A8HQ surface (Scheme 1). One last point about these electrochemical observations is that faradaic background currents are not completely eliminated upon Al(III) accumulation onto the surface because of non-zero

15

C = 4.62 F (corresponding to Γ = 17.39  10−12 mol 5A8HQ cm2 adsorbed on and intercalated in Au-MPA-5A8HQ) and C = 0.59 F (corresponding to Γ = 2.22  10−12 mol 5A8HQ cm2 intercalated in Au-MPA-5A8HQ-Al(III)). These data show that electroactive

free

5A8HQ

molecules

still

present

in

the

in-situ

prepared

Au-MPA-5A8HQ-Al(III) electrode. This is studied in detail in Section 3.3. Now that the presence of the intercalated and adsorbed 5A8HQ species is approved, one can calculate the equilibrium constant for the intercalation (int) as well as adsorption (ad) processes according to the following equilibria Γ

5A8HQ(aq) ⇋ 5A8HQ (int-in-SAM)

(int − in − SAM) =

Γ

C

C

Γ

5A8HQ(aq) ⇋ 5A8HQ (ad-on-SAM)

(ad − on − SAM) =

(1)

Γ

C

(2)

C

Considering C= 1 mol/L and  = 1.18210-9 mol/cm2 as the bulk and surface standard concentrations,

the

thermodynamic

int-in-SAM = 0.94 and further

theoretical

works

reaction

constants

are

thus

obtained

as

ad-on-SAM = 6.42. Basically, these data can be used for on

these

systems

and

for

optimization

of

the

Au-MPA-5A8HQ-Al(III) surfaces for experimental and application purposes.

3.2.3. Characterization by ATR-FTIR Typical ATR-FTIR spectra obtained on the surface of (a) bare Au, (b) Au-MPA, (c) Au-MPA-5A8HQ, and (d) the in-situ prepared Au-MPA-5A8HQ-Al(III) are demonstrated in Fig. 4 and discussed in this section. (a) The spectrum of the bare Au surface is featureless over the scanned frequency range, 1000-4000 cm−1 (curve a).

16

(b) The ATR-FTIR spectrum of the Au-MPA surface (Fig. 4, curve b) shows a band at 1680 cm1 which is attributed to the stretching of the C=O of the carboxylic carbonyl group [64] of the adsorbed MPA molecule. The broad peak appeared in the range of 2700-3570 cm1 in this spectrum is attributed to the O-H stretching of MPA carboxylic acid functions. (c) The intensity of the peaks appeared in the range of 600-3570 cm1 (curve b) is decreased significantly upon attachment of 5A8HQ (curve c) on Au-MPA. The MPA carbonyl group is converted to an amide carbonyl group which shows absorbance bands at 1650 cm1 shifted to a lower frequency in comparison with that of the MPA molecule (Fig. 4 curve b) [65]. In addition, the bands appeared at 1270 and 1130 cm−1 (Fig. 4, curve b) are assigned to a CH/CC=N bending + C=N stretching vibrations and the peak observed at 862 cm1 is attributed to the out-of-plane CH wagging vibrations [66,67]. The bands at 1423 and 1514 cm−1 correspond to C=C/C=N stretching + CH bending vibrations associated with both the pyridyl and phenyl groups in 8HQ. The vibrations involving C=C/C=N stretching + CH bending

of

the

quinoline

fragments

of

8HQ

are

observed

at

1340

and

1390 cm−1 (Fig. 4, curve c) [66,67]. (d) The observed bands in the ATR-FTIR spectrum of the Au-MPA-5A8HQ surface are almost disappeared by adsorption of Al(III) ions (Fig. 4, curve d). The effective decrease in the band intensities can be attributed to the bonding of Al(III) ion with the ligands immobilized on the electrode surface and fast stitching/sewing of the dangling dents of the surface ligand molecules by Al(III) ions. One point should be mentioned here. Since gas phase and droplet water molecules are eliminated in a vacuumed desiccator, strong peaks around 3500 cm-1 can only be attributed to the well-structured and adsorbed (trapped) water molecules on the Au surface (inside the SAM layers). Water molecules are oriented toward the surface in different ways [68], thus

17

resulting in different peak structures. The structure of the ice-like water layer on metal surfaces has already been well-studied and approved by means of computational methods [69-71] and several modern surface analytical techniques in UHV system [72,73]. The aforementioned results approve modification of the Au electrode surface by in-situ formation of Au-MPA-5A8HQ-Al(III), however the procedure needs to be improved to avoid intercalation and/or physical adsorption during modification of the Au-MPA surface by oxinate-Al(III) complex. This issue is also investigated in the following section.

3.3. Investigating the faradaic features superimposed on the backgrounds The observed electrochemical signal backgrounds at the surface of the in-situ prepared oxinate-Al(III) nanostructures as well as its superimposing faradaic features can be criticized for optoelectronic [18-22] and electronic [27,28] applications. Thus, we examined several routes to find appropriate method for construction of oxinate-Al(III) nanostructure on gold with minimum backgrounds, including optimizing the solvent effect (versus H2O), assembling of 5A8HQ:Al(III) by ex-situ method (versus in-situ), and linking spacers between the oxine and the gold surface.

3.3.1. Effect of organic solvent Depending on the proton donation degree of solvent (Section 1), the 8HQ molecule can exist in cis and/or trans forms (Supplementary Data, Fig. S3). The trans form can be the dominant conformation in the strong proton donor solvents such as H2O, favoring H-bond between the MPA-5A8HQ amide group as well as the top 5A8HQ groups of the Au-MPA-5A8HQ SAM and the free 5A8HQ, making intercalation and physical adsorption of free 5A8HQ into the SAM favorable when immobilization step is carried out in such a solvent (Supplementary Data, Fig. S4). To investigate this possibility, the covalent immobilization of 5A8HQ on the

18

Au-MPA electrode was repeated in DMF and DMF:CHCl3 solvent instead of water [74,75]. A comparison between the CVs obtained in the absence of any redox probe on the Au-MPA-5A8HQ SAM prepared in these two solvents (Fig. S5) shows that the 5A8HQ immobilized from DMF has significantly smaller background currents over the whole potential range. In DMF, the 8HQ moiety of the free 5A8HQ exists mainly in its cis-8HQ forms (Fig. S3), i.e., in its intra-molecular H-bond form [16,76], preventing physical adsorption of 5A8HQ. This is supported by the disappearance of the above-mentioned specific O-H stretching bands at wave numbers higher than 3000 cm−1 (Section 3.2.3) in the ATR-FTIR spectrum of the Au-MPA-5A8HQ(DMF) surface (Supplementary Data, Fig. S6). Therefore, it can be concluded that both the presence of the Au-MPA-5A8HQ amide group and solubility of ligand in water results in the ease entrance of 5A8HQ into the SAM, and thus, increases the CV background of the in-situ prepared Au-MPA-5A8HQ electrode. Another

examination

was

performed

by

sequential

immersion

of

as-prepared

Au-MPA-5A8HQ in n-hexane, water, and water containing 2 mM 5A8HQ, and then, recording the CVs of the Au-MPA-5A8HQ electrode in 0.05 M ABS, pH 5.5, 0.1 M NaClO4 in the absence of any external redox probes. Analysis of the CVs recorded in the clean buffer aqueous solution after each process (Fig. S7) revealed two interesting points: (i) the backgrounds decrease significantly by immersion of the as-prepared Au-MPA-5A8HQ surface in n-hexane, and (ii) the process is reversible, i.e. the backgrounds are restored by immersion of the cleaned (n-hexane washed) Au-MPA-5A8HQ in 2 mM 5A8HQ aqueous solution so that a behavior like as-prepared Au-MPA-5A8HQ is observed at the end. This treatment supports the presence of intercalated and physically adsorbed 5A8HQ in Au-MPA-5A8HQ.

19

However, a comparison between the behavior of Au-MPA-5A8HQ electrode after sequential treatment by n-hexane and water (Fig. S7, curves b and c) shows that water molecules, whether bounded to the amide groups (intercalated) or not, do not affect the backgrounds of the electrode. Although intercalation and physical adsorption of 5A8HQ is a drawback in preparation of pure Au-MPA-5A8HQ nanostructure in the present work, intercalation of inorganic and organic compounds into the layered nanostructure [77] have received significant attentions in recent years owing to their applications in various fields such as electrocatalysis [78], energy-storage applications [79], sensors [80], rechargeable batteries [81], solar cells [82], photochemical redox reactions [83], ion exchange processes [84] and synthesis of composite materials [85]. Therefore, advantage or disadvantage of the intercalation of a species into a nanostructure SAM depends on the application the nanostructure is intended for.

3.3.2. Incubation (treatment) in HNO3 solution A washing treatment (incubation) of Au-MPA-5A8HQ within 0.10 M HNO3 resulted in a significant decrease in the electrochemical response backgrounds. Most probably, protonation of moieties involving in intercalation and physical adsorption decreases the chance of H-bond and remove effectively the physically adsorbed 5A8HQ from the Au-MPA-5A8HQ electrode surface. This behavior supports further intercalation and physical adsorption of 5A8HQ on Au-MPA-5A8HQ electrode (Supplementary Data, Fig. S8).

3.3.3. Effect of linking spacer Nature of the spacers plays an important role in the intercalation of biological and organic molecules inside the thin films formed on the surface. The intercalated electroactive species can establish a faradaic contribution to the electrode process. This issue can help here to trace

20

the nature of the interaction of intercalated 5A8HQ with Au modified surface. Accordingly, two types of spacers, azo group [14,86] and amide group, were selected, bearing the 8HQ functions onto the Au-SAM surface (Supplementary Data, Fig. S9). The results (Fig. 5) indicate that the electrochemical signal backgrounds observed on the Au electrode modified by oxinate functional group depends highly on the nature of the spacers, so that the backgrounds were minimized upon using azo spacer [14,86] formed by ATP and maximized upon using two amide spacers formed by glycine [87] (Supplementary Data, Fig. S9). This comparative trend approves our conclusion about the role of H-bond in the intercalation of 5A8HQ into the Au-MPA-5A8HQ SAM discussed in the previous section. 3.3.4. The ex-situ approach Preparation and attachment of 5A8HQ:Al(III) complex via an ex-situ approach may allow one to decrease concentration of the free 5A8HQ and better control the behavior of Au-MPA-5A8HQ-Al(III) structure. Accordingly, mixtures of 5A8HQ and Al(III) solutions with different 5A8HQ:Al(III) ratios were prepared to form the 5A8HQ:Al(III) complex. The ratio of 1:3 was found to be the optimum ratio, ensuring minimum free 5A8HQ left in the immobilization solution. As discussed above, the free 5A8HQs can intercalate into and be physically adsorbed onto the Au-MPA-5A8HQ layer via bonding to the amide groups which then result in an increase in the background currents. In addition, the NH2 group of the free 5A8HQ can compete with the NH2 groups of the 5A8HQ:Al(III) for attachment to the activated COOH group of the Au-MPA surface, resulting in a mixture of Au-MPA-5A8HQ and Au-MPA-5A8HQ:Al(III) arrays on the surface which is not desirable. Immobilization of the ex-situ formed 5A8HQ:Al(III), onto the Au-MPA is traced via the following approaches to better understand the nature of the Au-MPA-5A8HQ-Al(III) structure and electrochemical behavior.

21

3.3.4.1. Voltammetry and EIS in the absence of any redox probe The

CVs

obtained

during

step-by-step

ex-situ

formation

(Scheme 1)

of

the

Au-MPA-5A8HQ:Al(III) in the absence of any redox probe (Fig. 6) show that the backgrounds are decreased significantly after each step so that smooth featureless CV is obtained for the Au-MPA-5A8HQ:Al(III) surface (Fig. 6, curve c). One may compare these results with the large backgrounds (opposite behavior) observed on Au-MPA-5A8HQ-Al(III) surface prepared via in-situ method (Fig. 3A, curves b and c). This interesting behavior implies that the intercalation and physical adsorption of 5A8HQ have now negligible contributions in the structure of the ex-situ prepared Au-MPA-5A8HQ:Al(III) surface. Analysis of the EIS complex plane plots, recorded for the Au-MPA-5A8HQ:Al(III) surface (Fig. 6B) did not reveal a significant value for C, supporting the above findings based on the CV. 3.3.4.2. Voltammetry and EIS in the presence of redox probes Attachment of the ex-situ prepared 5A8HQ:Al(III) complex onto the Au-MPA surface is investigated by CV,DPV and EIS methods based on the redox reaction rates of [Fe(CN)6]3– and FcMeOH probes (Supplementary Data, Figs. S10 and S11). Analysis of the data obtained in this investigation indicates a good agreement between voltammetric currents (Ips) and the EIS data (Tables S1 and S2). These results imply that 5A8HQ:Al(III) is transferred onto the Au-MPA, and thus, a compact and dense film is formed on the surface which impedes the electron transfer between the Au surface and the redox probe.

3.3.4.3. ATR-FTIR spectroscopy Attachment of the ex-situ prepared 5A8HQ:Al(III) complex onto the Au-MPA SAM electrode is properly confirmed by ATR-FTIR (Fig. 7). The ATR-FTIR spectrum of the ex-situ prepared Au-MPA-5A8HQ:Al(III) (spectrum b) showed a sharp band [88] at 1450 cm-1 in

22

comparison with the crowded spectrum with many overlapping bands obtained for the Au-MPA-5A8HQ electrode appeared over the 1400-1600 cm−1 range (spectrum a). A comparison

between

ATR-FTIR

spectra

of

the

in-situ

and

ex-situ

prepared

Au-MPA-5A8HQ-Al(III) surfaces (spectra a and b) suggests also that the absorption bands of ex-situ prepared Au-MPA-5A8HQ:Al(III) surface are relatively intense and more resolved than the weak and structureless bands of the in-situ prepared Au-MPA-5A8HQ-Al(III) surface. 3.3.5. Surface saturation by ligands (incubation in 8HQ and 5A8HQ solutions) One question which may be arisen here is that if there is any open (free coordination) site available at the Au-MPA-5A8Q-Al(III) electrode. The following experiments are carried out to address this question. Two sets of Au-MPA-5A8HQ-Al(III) electrodes, prepared by in-situ and ex-situ respectively, were exposed to the 0.1 M 8HQ solution prepared in ethanol, and then, their voltammetric behavior was studied in the presence of the FcMeOH probe. The voltammogram Ip obtained for the FcMeOH probe on the in-situ modified surface undergoes a large variation upon exposure to the 0.1 M 8HQ solution (Fig. 8 A, compare curves a and b), but on the ex-situ modified surface, the Ip remains almost invariant (Fig. 8 B). Since the FcMeOH is an uncharged probe, the decrease observed in its Ip (Fig. 8 A) is related to the

impeding of the surface

by

adsorption of 8HQ

on in-situ prepared

Au-MPA-5A8HQ-Al(III) electrode, which suggests presence of some uncoordinated Al(III) sites on the Au-MPA-5A8HQ-Al(III) electrode surface having coordination capacity towards the incoming 8HQ. Since the ex-situ prepared Au-MPA-5A8HQ:Al(III) electrode has not shown such changes, complexation of Al(III) via this method is highly ordered and most probably having a complete 1:3 stoichiometric ratio for Al(III):5A8HQ as Al[5A8HQ]3. This is one of the interesting results of this work.

23

The voltammetric results are supported by ATR-FTIR signals (Fig. 8 C and D, curves a and b). The ATR-FTIR obtained for accumulation of 8HQ on in-situ prepared Au-MPA-5A8HQ-Al(III) shows that intensity of the peaks at the 1300-1700 cm1 range related to quinoline and phenol groups of 8HQ is increased in comparison with that in the spectrum of the intact Au-MPA-5A8HQ-Al(III) surface which is in accordance with what is reported in literature [88]. Based on the analysis of the results obtained from ATR-FTIR, it can be said that the 8HQ accumulates on in-situ prepared Au-MPA-5A8HQ-Al(III), implying that complexes with different ratios and/or non-ordered complexes are formed in this method, unlike the complete coordination and ordered structure of the 5A8HQ:Al(III) layer obtained via the ex-situ method (Scheme 1, only few of the possible structures are presented for in-situ method). 3.4. Surface concentrations of the Au-MPA-5A8HQ-Al(III) components To address surface concentrations (i) of the Au-MPA-5A8HQ-Al(III) components, a complete set of X-ray photoelectron spectroscopy (XPS) measurements are carried out on the Au-MPA, Au-MPA-5A8HQ and in-situ prepared Au-MPA-5A8HQ-Al(III) surfaces, as an example (Fig. 9). From the analysis of the results (Table S3), the relative stoichiometry of MPA : 5A8HQ : Al(III) = 5.7 : 2 : 1.04 is obtained (which can be considered to be 6 : 2 : 1). Since the Al(III) ions should complete their coordination vacancies, the stoichiometry of 2:1 obtained for 5A8HQ:Al(III) does not seem to be confident. To explain this stoichiometry, the following suppositions may be made. (1) Some of the Al(III) ions may be bridged between two neighboring 5A8HQ moieties. (2) Some of the Al(III) sites may have been remained uncoordinated on the in-situ prepared Au-MPA-5A8HQ-Al(III) electrode. However, assumption (2) is in good agreement with the conclusion drown in Section 3.3.5, that is “adsorption of 8HQ on in-situ prepared Au-MPA-5A8HQ-Al(III) electrode, which suggests

24

presence of some uncoordinated sites on the Au-MPA-5A8HQ-Al(III) electrode surface having coordination capacity towards the incoming 8HQ”. Furthermore, from the voltammetry desorption experiments [52] carried out on the Au-MPA surface, a value of MPA = 7.541010 mole cm2 is obtained for the surface coverage of MPA. Combining this voltammetry data with atomic ratios obtained from XPS experiment, results in the values of MPA = 7.541010, 5A8HQ = 2.651010 and Al(III) = 1.321010 for the surface concentrations of MPA, 5A8HQ and Al(III).

3.5. Reproducibility,

stability

against

electrolyte

solutions

and

durability

(stability) against time (i) Reproducibility: Reproducibility shows the degree of agreement of measurements on the replicate devices, so it is assessed by means of precisions. Here, reproducibly is measured in terms of the agreements between responses of a group of modified electrodes (devices), prepared by the same method and applied for the same purposes. The reproducibility of the Au–MPA-5A8HQ-Al(III) electrode is verified as follows: a set of clean Au electrodes was prepared and modified to form Au–MPA-5A8HQ electrodes. The modified electrodes were used for accumulation of Al(III) from 1.0×10-5 M Al(III) solution. Then, the response of each electrode was measured individually against the redox reaction of FcMeOH in the same conditions. The results obtained by DPV for three in-situ and ex-situ prepared Au-MPA-5A8HQ-Al(III) electrodes (n = 3) showed mean relative standard deviations of 11% and 10%, respectively. Overall, the results support reproducibility of the prepared electrodes; however, reproducibility of the ex-situ prepared electrode is slightly better than that of the in-situ prepared electrode. (ii) Stability against electrolyte solutions: Stability of Au-MPA-5A8HQ-Al(III) is also tested against

several

electrolytes,

and

the

following

25

results

are

obtained.

(1) The

Au-MPA-5A8HQ-Al(III) electrode is stable in neutral electrolytes like KCl, NaClO4 for long times. (2) The Au, Au-MPA, and Au-MPA-5A8HQ electrodes are stable in phosphate based electrolytes and buffers, however, phosphate ion is adsorbed on the Au-MPA-5A8HQ-Al(III) electrode during electrochemical (CV and EIS) measurements carried out in phosphate buffers if experiments are continued for more than a few hours. It means that the AuMPA-5A8HQ-Al(III) does not show stable behavior in phosphate solutions. Accordingly, electrochemical studies related to this electrode (Au-MPA-5A8HQ-Al(III)) have been performed in acetic acid/acetate buffer solution. (3) The Al(III) ions can be removed from the Au-MPA-5A8HQ-Al(III) surface by using 0.1 M NaF solution, pH 5.0. Therefore, this solution is used for recovery of the Al(III) ion from the Au-MPA-5A8HQ-Al(III) surface. (4) As mentioned in the Section 3.3.5, the 8HQ molecules are adsorbed on the in-situ prepared Au-MPA-5A8HQ-Al(III) electrode, traced by CV and EIS in the presence of the FcMeOH redox probe. However, the case is different for ex-situ prepared Au-MPA-5A8HQ:Al(III) electrode. While these

electrodes

enjoy

the

stabilities

mentioned

for

the

in-situ

prepared

Au-MPA-5A8HQ:Al(III) electrode, they resist against adsorption of 8HQ, supporting complete complexation of Al(III) in ex-situ prepared Au-MPA-5A8HQ:Al(III) electrode stricter. (iii) Durability

(stability)

against

time:

The

in-situ

and

ex-situ

prepared

Au-MPA-5A8HQ-Al(III) electrodes were stored in clean atmosphere at ambient temperature (kept in a vial) when they were not in use. The long term storage stability of the electrodes was tested by monitoring the anodic peak current of FcMeOH measured by DPV after different storage times. The tests were repeated almost every two days up to a total of 15 days. Decreases of ~5.6% and 2.2% were observed for in-situ and ex-situ approaches after about 15 days, implying good stability for these systems. It means that while both ex-situ and in-situ

26

prepared Au-MPA-5A8HQ-Al(III) electrodes are stable for long times, the ex-situ prepared electrodes are more stable.

4. Conclusion Oxinate-aluminum nanostructure assemblies based on attachment of 5A8HQ on the gold surface modified with MPA, Au-MPA-5A8HQ-Al(III), were constructed for the first time. Physicochemical characterization of the Au-MPA-5A8HQ-Al(III) nanostructures, were carried out by CV, DPV and EIS electrochemical techniques in the absence and presence of different redox probes, [Fe(CN)6]3/4 and FcMeOH, and supported by ATR-FTIR and XPS measurements. The results showed that oxinate-Al(III) structure had been successfully constructed on Au–MPA through both in-situ and ex-situ methods, but with different spectroscopic and electrochemical features. Following the immobilization stages of the in-situ approach revealed unconventional features in the electrochemical signals and backgrounds of the surface. In order to find the sources of these features and establish appropriate method for construction of high purity oxinate-Al(III) nanostructures on gold with minimum electrochemical backgrounds, several routes were examined including (i) use of different solvents (DMF and DMF:CHCl3 versus H2O), (ii) acid (HNO3) treatment of the prepared structures, (iii) use of different linking spacers between oxinate-Al(III)

and

gold

surface,

(iv) ex-situ

preparation

and

study

of

Au-MPA-5A8HQ:Al(III) in comparison with the in-situ prepared Au-MPA-5A8HQ-Al(III) modified surfaces, and (v) comparative study of the physicochemical behavior of the in-situ and ex-situ modified surfaces before and after incubation in 8HQ solution (saturation with ligands). The results showed that non-ordered structures with different ratios of oxinate:Al(III) and large

amount

of

physically

adsorbed

27

and

intercalated

5A8HQ

inside

the

Au-MPA-5A8HQ-Al(III) were formed by in-situ method. While, highly ordered Au-MPA-5AH8Q:Al(III) structure with completely coordinated Al(III) (almost without physically adsorbed or intercalated 5A8HQ) were formed via ex-situ method so that this structure can be denoted as Au-MPA-[(5A8HQ)Al(III)(5A8HQ)2]. It is thus concluded that highly stable, reproducible, compact and pure oxinate-Al(III) structure could be formed on gold via ex-situ method. Finally, the XPS measurements allowed assessing the presence, the stoichiometry and the oxidation states of the elements in the film components. Solid state studies [23,89], such as determination of the crystallographic structure and light emitting efficiency of the prepared Al[5A8HQ]3 complexes, are in order as interesting topics of research, which can be regarded as new challenges in their development for applied purposes.

Acknowledgement The authors gratefully acknowledge the University of Isfahan for financial supports and research facilities.

28

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36

crystalline

tris(8-

Al MQMAS

NMR

Figure Captions Scheme 1. A schematic representation of the proposed oxination mechanism via activation of carboxylic acid groups on the Au-MPA SAM by EDC/NHS and accumulation (covalent-coordination) of Al(III) via two in-situ and ex-situ approaches. Incubation of the in-situ prepared Au-MPA-5A8HQ-Al(III) in 0.1 M 8HQ ethanol solution is also demonstrated. Only few of the possible structures are presented for in-situ method. Fig. 1.

Tracing step-by-step formation of Au-MPA-5A8HQ-Al(III) via in-situ approach using [Fe(CN)6]3−/4 redox probe. The CVs (A), DPV (B), and the EIS complex plane plots (C) obtained on the Au (a), Au-MPA (b) and Au-MPA-5A8HQ SAM electrodes before (c) and after (d) immersion in 1.0×10−5 M Al(III) at pH 5.0 for 15 min. The scan rate is 100 mVs−1 for CVs. Measurement solution conditions: 0.05 M ABS, pH 5.5, containing 0.1 M NaClO4 in the presence of 0.5 mM [Fe(CN)6]3−. Frequency range, 100 kHz to 0.1 Hz, EDC = 200 mV vs. Ag/AgCl. Inset of panel (C) shows the CPE equivalent circuit model (M1) used for fitting of the EIS data. The experimental data are shown as symbols and the fitted results as continuous lines (a) to (d).

Fig. 2.

Step-by-step formation of Au-MPA-5A8HQ-Al(III) via in-situ approach traced by using 0.5 mM FcMeOH neutral (uncharged) redox probe. The CVs (A), DPV (B), and EIS complex plane plots (C) obtained on the Au (a), Au-MPA SAM (b) and Au-MPA-5A8HQ SAM electrodes before (c) and after (d) immersion in 1.0 105 M Al(III) at pH 5.5 for 15 min. Other conditions are the same as for Fig. 1.

Fig. 3.

(A) CVs and (B) EIS complex plane plots obtained in the absence of external probe on the (a) Au, (b) Au-MPA, (c) Au-MPA-5A8HQ and (d) in-situ prepared Au-MPA-5A8HQ-Al(III) surfaces. Insets of panel (B) show the two equivalent

37

circuit models used for fitting the EIS data, M2 for curves (a) and (b); and M3 for curves (c) and (d). Other conditions are the same as for Fig. 1. Fig. 4.

The ATR-FTIR spectra of the (a) bare Au, (b) Au-MPA, (c) Au-MPA-5A8HQ and (d) Au-MPA-5A8HQ-Al(III) SAM electrodes prepared via in-situ method.

Fig. 5.

The CVs obtained in the absence of any external probe in 0.05 M ABS, pH 5.5, containing 0.1 M NaClO4 during oxination of Au surface via in-situ approach using two different types of linking spacers: (A) the N=N spacers: (a) Au, (b) Au-ATP, and (c) Au-ATP-8HQ SAM electrodes, and (B) amide spacers (-CONH-CH2-CO-NH-):

(a) Au,

(b) Au-MPA,

(c) Au-MPA-Gly,

and

(d) Au-MPA-Gly-5A8HQ electrodes. Compare curve (c) of panel (A) with curve (d) of panel (B). For a schematic representation of the proposed mechanism see Supplementary Data, Fig. 9. Fig. 6.

Tracing step-by-step formation of Au-MPA-5A8HQ:Al(III) electrode via ex-situ approach in the absence of any redox probe: (a) Au, (b) Au-MPA SAM, and (c) Au-MPA-5A8HQ:Al(III) SAM electrodes. Measurement conditions are the same as given in Fig. 1.

Fig. 7.

A comparison between the ATR-FTIR spectra of 5A8HQ complexes of Al(III) prepared on the Au-MPA surface via (a) in-situ and (b) ex-situ methods.

Fig. 8.

The CVs (A,B) and ATR-FTIR spectra (C,D) obtained in the presence of FcMeOH

as

an

external

probe

Au-MPA-5A8HQ-Al(III) (A,C) Au-MPA-5A8HQ:Al(III) (B,D)

on

and SAM

electrodes

the

in-situ

ex-situ before (a)

prepared prepared and after (b)

immersion in 0.1 M 8HQ ethanol solution for 15 min. Measurement conditions are the same as given in Fig. 1. Fig. 9.

Survey XPS spectrum obtained on the in-situ prepared Au-MPA-5A8HQ-Al(III) electrode from 0 to 600 eV. The calibration is based on C1s peak at 284.7 eV.

38

Tables

Table 1. Electrochemical parameters extracted from the EIS data (Fig. 1) obtained during step-by-step modification of gold electrode toward the in-situ prepared Au-MPA-5A8HQ-Al(III) SAM electrode, in 0.05 M ABS, pH 5.5, containing 0.1 M NaClO4 in the presence of 0.5 mM [Fe(CN)6]3- (Direct current potential, EDC = +200 mV). kapp,het Rs Rct Q Cdl Electrode g (kΩ) (kΩ) (10−6 sgΩ−1) (×106 s1) (µF) Au

0.11 ± 0.22

0.55 ± 0.01

6.07 ± 5.83

0.92 ± 0.71

960.29 ± 0.20

3.08

Au-MPA

0.09 ± 0.01

46.64± 0.06

1.85 ± 0.38

0.96 ± 0.01

11.41 ± 0.01

1.24

Au-MPA-5A8HQ

0.09 ± 0.01

21.08 ± 0.02

2.04 ± 1.05

0.96 ± 0.01

25.23 ± 0.01

1.46

Au-MPA-5A8HQ-Al(III)

0.09 ± 0.01

10.24 ± 0.01

2.15 ± 0.70

0.96 ± 0.01

51.95 ± 0.06

1.43

The constant phase element (CPE) model (Fig. 1C, inset Model M1) was enough to explain the experimental EIS data, in which ZCPE was replaced for impedance of double layer capacitance (Cdl), where ZCPE = (Q(j)g)1, Q is double layer parameter, j = (1)1/2,  = 2f, f is frequency of the EAC, the 'g' parameter is dimensionless related to the rotation of the complex plane plot, which in turn is assumed to be connected to the electrode surface inhomogeneity, so, for ideal smooth electrodes g = 1. Also, Q = Cdlg [Rs1+Rct1]1g, the Rs and Rct are solution and charge transfer resistances, so Q = Cdl for smooth electrodes, kapp,het is apparent heterogeneous charge transfer rate constant, where kapp,het = RT(n2F2RctC)1, and C is concentration of the electroactive diffusing species in the solution phase, [Fe(CN)6]3-/4-. For detailed analysis of the EIS data see ref 34. The errors of electrochemical data are errors related to the fits, obtained using Z-View program and appropriate models built in.

Table 2. The same as Table 1, but in the presence of FcMeOH (Fig. 2). See footnote of Table 1 for the definition of parameters. kapp,het Rs Rct Q Cdl Electrode g 6 1 (kΩ) (kΩ) (10−6sgΩ−1) (µF) (×10 s ) Au

0.14±0.02

0.003±0.01

75.25±2.57

0.83±0.36

177.33 ± 5.91

0.89

Au-MPA

0.23±0.01

0.42±0.02

18.83±5.56

0.85±0.01

1.27±0.14

0.47

Au-MPA-5A8HQ

0.24±0.03

2.97±0.01

23.55±5.03

0.76±0.87

0.18±0.03

0.21

Au-MPA-5A8HQ-Al(III)

0.34±0.02

3.81±0.01

18.87±2.41

0.79±0.42

0.14±0.01

0.27

See footnotes of Table 1.

39

Table 3. Electrochemical parameters extracted from the EIS data (Fig. 3B), obtained during step-by-step modification of gold electrode toward in-situ prepared Au-MPA-5A8HQ-Al(III) SAM electrode, in 0.05 M ABS, pH 5.5, containing 0.1 M NaClO4 (EDC = +200 mV), in the absence of any redox probe. Electrode Rs Q1 g Cdl Rct Q2 Cφ Kapp,hom −6 g -1 (kΩ) (10 s Ω ) (µF) (MΩ) (µF) (s−1) Au

0.14±0.03

2.90±0.01

0.91±0.01

1.33

NA

NA

NA

NA

Au-MPA

0.19±2.37

2.15±0.22

0.89±0.47

0.82

NA

NA

NA

NA

Au-MPA-5A8HQ

0.20±0.06

2.08±0.01

0.98±0.01

1.77

0.11±0.01

5.31±0.01

4.62

0.98

Au-MPA-5A8HQ-Al(III)

0.12±0.01

1.80±1.11

0.91±0.01

0.75

0.77±0.02

1.39±0.01

0.59

1.10

A simple model (Fig. 5C, inset Model M2), including a CPE in series with Rs, was enough to explain the EIS data of Au and Au-MPA electrodes. However, the Au-MPA-5A8HQ and Au-MPA-5A8HQ-Al(III) electrodes showed a faradaic behavior, and a modified Randles’ model (Fig. 5C, inset, Model M3) was used to explain their EIS data. The impedance of CPE is given as ZCPE = (Qi(jω)g)−1, Qi = Cig [Rs−1 + Rct−1]1−g, where Q1 and Q2 are model parameters related to the capacitance (C1 = Cdl) and pseudocapacitance (C2 = Cφ) parameters. The Q1 = Cdl and Q2 = Cφ for smooth electrodes. The Cφ stands for confined redox reaction (adsorbed 5A8HQ electroactive species), kapp,hom is apparent homogeneous electron transfer rate constant, where kapp,hom = (2RctCφ)−1. Other parameters are the same as introduced in Table 1. NA: Not applicable. See footnotes of Table 1.

40

Scheme & Figures

Scheme 1.

41

Fig. 1.

42

Fig. 2.

43

Fig. 3.

Fig. 4.

44

Fig. 5.

Fig. 6.

45

Fig. 7.

46

Fig. 8 10000 Au 4f Au 4f

Intensity(cps)

8000

6000

Au 4d

C 1s

N 1s

4000

O 1s Au 4p

S 2p

Al 2p3/2

2000

Au 4p 0 0

100

200

300

Bonding energy/eV

Fig. 9.

47

400

500

600