Conjugates of calixarenes emerging as molecular entities of nanoscience

Coordination Chemistry Reviews 256 (2012) 2096–2125

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Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Conjugates of calixarenes emerging as molecular entities of nanoscience Amitabha Acharya, Kushal Samanta, Chebrolu Pulla Rao ∗ Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India

Contents 1. 2.

3.

4.

5.

6.

7. 8.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solid lipid nanoparticles (SLNs) of calix[n]arenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Upper rim, long chain alkyl conjugates of calix[n]arenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Effect of carbohydrates and ionic strength of salts on CSLNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Adsorption of proteins and DNA on the surface of CSLNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Small molecular adsorption on the surface of CSLNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Toxicity effect of CSLNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Calix[n]arene based gold/silver NPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Detection of PAHs by AgNPs coated with the conjugates calix[4]arene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. AgNPs of amino-, phosphonato- and thia-calixarenes and their applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. NPs of the conjugates of calixarenes by direct synthesis and/or post-synthetic surface modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Double rosette assemblies of NPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . para-Sulfonated calixarene for NP/SWCNT preparations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Magnetic nanoparticles stabilized by normal and sulfonated conjugates of calixarenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. SWCNTs protected by the water soluble sulfonated and phosphonated calixarenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. AgNPs of the water soluble conjugates of calixarenes and their applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. AuNPs of the water soluble conjugates of calixarenes and their applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Cu2 ONPs of the water soluble conjugates of calixarenes and their applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Calix[n]arene QDs and other NPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. QDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1. QDs of sulfonated calix[4]arene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2. Host–guest complexes of QDs and core shell QDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.3. Detection of pesticide and PAH by QDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Other NPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1. TiO2 nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2. Magnetic nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Guest species (metal ion/amino acid/protein) induced microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Hg2+ as guest species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Cu2+ as guest species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Ag+ as guest species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4. Zn2+ as guest species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5. Zn2+ followed by amino acids and proteins as guest species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6. Carboxylate side chains in Asp, Glu and proteins as guest species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bio-molecular interactions with the conjugates of calixarenes by microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and future directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2097 2097 2097 2099 2099 2101 2101 2101 2101 2102 2106 2107 2107 2107 2108 2110 2110 2111 2111 2111 2112 2112 2112 2115 2115 2115 2117 2118 2119 2120 2120 2120 2121 2122 2124 2124 2124

Abbreviations: 13 C CP MAS NMR, 13 C cross polarization and magic angle spinning nuclear magnetic resonance; AFM, atomic force microscopy; AgNP, silver nanoparticle; AuNP, gold nanoparticle; BAM, Brewster angle microscopy; CD, circular dichroism; CSLNS, calixarene solid lipid nanoparticles; CV, cyclic voltammetry; DLS, dynamic light scattering; DPV, differential pulse voltammetry; DRIFT, diffuse reflectance infrared Fourier transform spectroscopy; DSC, differential scanning calorimetry; DTA, differential thermal analysis; EDX, energy-dispersive X-ray; FCS, fluorescence correlation spectroscopy; FFT, fast Fourier transform; FTIR, Fourier transform infrared; G, guest; GFP, green fluorescent protein; GS, gemini surfactant; HRTEM, high resolution transmission electron microscopy; LB films, Langmuir–Blodgett films; LbL, layer by layer; LMCT, ligand to metal charge transfer; MNP, magnetic nanoparticles; MPC, monolayer protected clusters; NMR, nuclear magnetic resonance; NP, nanoparticles; PAH, polycyclic aromatic hydrocarbons; PCS, photon correlation spectroscopy; Powder XRD, powder X-ray diffraction; QD, quantum dots; SAED, selected area of electron diffraction; SAM, self assembled monolayer; SE, spectroscopic ellipsometry; SEM, scanning electron microscopy; SERS, surface enhanced Raman spectroscopy; SLN, solid lipid nanoparticle; SPR, surface plasmon resonance; SQUID, superconducting quantum interference device; SWCNT, single wall carbon nanotube; TEM, transmission electron microscopy; TGA, thermal gravimetric analysis; UV–vis spectra, ultra violet–visible spectra. ∗ Corresponding author. Tel.: +91 22 2576 7162; fax: +91 22 2572 3480. E-mail address: [email protected] (C.P. Rao). 0010-8545/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ccr.2012.05.018

A. Acharya et al. / Coordination Chemistry Reviews 256 (2012) 2096–2125

a r t i c l e

i n f o

Article history: Received 16 February 2012 Accepted 14 May 2012 Available online 29 May 2012 Keywords: Conjugates of calixarenes Solid lipid nanoparticles Magnetic nanoparticles Quantum dots Single wall carbon nanotube High resolution transmission electron microscopy

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a b s t r a c t Calixarenes, along with cyclodextrins (viz. ␤-CD) and crown ethers, are the most commonly studied macrocyclic compounds. These bucket shaped molecules have gained tremendous importance for having both hydrophilic and hydrophobic compartments together in the same species. The primary target of this review is to give the readers of various disciplines an updated overview of the recent advancements of the conjugates of calixarenes with respect to their nano chemistry in understanding the nanoscience of these supramolecular systems. Calixarenes now compete with cyclodextrins and calixresorcinols. Thus it is important to look into the current advancement in the literature to draw appropriate comparisons and to provide appropriate designs for their nano science aspects. This review is expected to provide all this. The review covers over 113 conjugates of calixarenes published in 106 references that include contributions from our research group. All the conjugates and their nanoscience aspects were depicted through 63 figures and two schemes. The characterization of these nano systems has been carried out by a variety of techniques that includes spectroscopy and microscopy. The advancement of the conjugates of calixarenes in the nanoworld can be considered as one of the most recent encroachment in the calixarene chemistry and is expected to come out as a major thrust research area in near future. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The physical or chemical properties of bulk materials can significantly improve or radically change when their size comes down to nanometer scale owing to the availability of more reactive centers on the surface of such nanoparticles (NPs) as compared to the bulk [1]. A few examples of NPs are, colloidal gold and silver [2,3], iron oxide crystals [4,5], semiconductor quantum dots (QDs) [6,7], which are present in 1–20 nm size. For synthesizing NPs, generally either the top-down or the bottom-up approach was used with the help of high intensity laser beams, chemical methods and thermolysis being most common. Calixarenes, along with cyclodextrins (viz. ␤-CD) and crown ethers, are the most commonly studied macrocyclic compounds [8,9]. These bucket shaped molecules are very important because (i) they possess both hydrophilic and hydrophobic compartments together in the same molecule, (ii) they are easy to prepare in large quantities, (iii) of the availability of easy and simple methods for selective derivatization, (iv) of the conformational flexibility of the platform as well as the derivatized arms, (v) of binding group tunability, (vi) of the variability in cavity size, etc. For quite some time, conjugates of calixarenes have been used for molecular recognition, as biosensors, as catalysts, as biomimetic models, and as templates for forming supramolecular aggregates. With these advancements the calixarenes now take the position of the molecular entities of the world of nanoscience and nanotechnology. The rapid growth of the involvement of calixarene systems for their use in nanoscience and nanotechnology can also be attributed to their non-toxicity, non-immunogenicity, and their chemical and biological stability, besides their easy production by known methodologies. Further, the conjugates of calixarenes can be good mimics for a wide range of natural substances including proteins and nucleic acids, which allows their use in biological systems. In recent years, our group has been working on ion and molecular recognition of the conjugates of calixarenes and our contributions in this area have been recently reviewed [10]. We have further targeted our focus at the field of calixarenes to study the microscopic architectural changes of these conjugates upon recognition by the guest species, and preliminary results of these were published. Though there are several reviews on calixarene recognition or calixarene self-assembly process, there is practically no review available which includes all types of calixarene NPs and QDs highlighting their nanoscience features, therefore, the current review. In the literature, different types of calixarene NPs are known, viz. calixarene based SLNs, Au/Ag NPs, QDs and magnetic NPs and TiO2 based NPs. Though different types of CH2 bridged calixarene derivatives have been included, thia calixarene and

calix–resorcinols are not included owing to their differences in chemical properties from the main calixarene family. Apart from these applications, calixarene conjugates have also been studied for other purposes, viz. (a) calixarene–biomolecular interactions by microscopy, (b) guest species induced microscopy features, (c) thin film coating, (d) self-assembly on substrate surface to generate different architectures, and (e) self-assembled monolayer (SAM) formation on Au(1 1 1) surface, etc. Only (a) and (b) are being covered in this review, since (c)–(e) have been recently reviewed partly in the literature [11]. This review proposes to cover all the publications in this direction during the period, 2000–2011, dealing with different conjugates of calixarenes used for different NP and QD preparations. The characterization of these NPs utilizes one or more of the following techniques, viz. AFM, CD, CV, DLS, DRIFT spectroscopy, EDX, FTIR, NMR, PCS, UV–vis, spectroscopy, powder XRD, spectroscopic ellipsometry, SEM, SPR and TEM, and the results were accordingly discussed. Thus it covers various types of calixarene NPs and QDs, viz. solid lipid nanoparticles (SLNs), magnetic nanoparticles, Ag/Au nanoparticles, TiO2 nanoparticles and quantum dots. Calixarenes are a class of versatile macrocyclic compounds that can be prepared by base catalyzed condensation of para-alkyl phenol and formaldehyde as shown in Scheme 1. The number of aromatic rings present in the oligomer is determined by the type of base used. Generally this type of supramolecular moiety looks like a flower vase or calix and it consists of several arene rings, that is why these are named as calix[n]arenes, where ‘n’ is the number of the arene moieties present in the structure. 2. Solid lipid nanoparticles (SLNs) of calix[n]arenes Para-acylcalix[4]arenes can be synthesized from de-alkylated calix[n]arenes via Friedel Craft’s acylation. This class of molecules can form self-assembled dimeric nanocapsules through van der Waals forces, Langmuir–Blodgett (LB) films on the substrate surface, solid lipid nanoparticles (SLNs) which themselves can selfassemble to generate larger structures, etc. These nanocapsules can selectively capture and release various guest materials and can also act as nanoreactors for a variety of chemical reactions [12]. 2.1. Upper rim, long chain alkyl conjugates of calix[n]arenes Amphiphilic calix[4]arene conjugates, 3–6, form stable monolayers, LB layers and SLNs (Fig. 1). The LB films that were prepared at lower surface pressure yielded “crater-like” structures of variable diameter. But the absence of such structures at higher pressure establishes the fact that there were persistent expanded liquid

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R

HO OH R

OH

HO

R

HO-

+

CH2O

OH

n R

R

p-tert-butyl calix[n]arena (1) R = C(CH3)3; n = 1; 1a R = C(CH3)3; n = 2; 1a R = C(CH3)3; n = 3; 1c R = C(CH3)3; n = 4; 1d R = C(CH3)3; n = 5; 1e

n=1-5

de-alkylated calix[n]arene (2) R = H; n = 1; 2a R = H; n = 2; 2b R = H; n = 3; 2c R = H; n = 4; 2d R = H; n = 5; 2e

Scheme 1. Cyclic condensation of p-tert-butyl phenol with formaldehyde resulting in the formation of calixarenes (1a–1e). Compounds 2a–2e are prepared by dealkylating the tert-butyl moieties of 1a–1d using AlCl3 . Ring size can be controlled by varying the reaction conditions.

domains, in the condensed liquid film. The SLNs have been prepared by interfacial solvent displacement method. The characterization was carried out by using photon correlation spectroscopy (PCS) and atomic force microscopy (AFM) and the sizes of the particles were in the range of ∼190 nm with height of ∼90 nm [13]. Calixarene based SLNs (CSLNs) (3) are good carriers for cosmetically important substances, such as, trans-2-ethylhexyl4-methoxy-cinnamate (t-EHMC). The comparison of the powder XRD patterns reveal that t-EHMC induces ordering of the 3 based CSLNs into microcrystalline material similar to 3 where t-EHMC is entrapped in the capsule formed by two calixarenes. These were

further confirmed by 13 C CP MAS NMR spectra [14]. Similar kinds of studies have also been carried out with 3 based CSLNs and 4-methoxy-2,2,6,6-tetramethylpiperidine-N-oxyl (MT), where MT unit were trapped [15]. These amphiphilic calixarene (3) nanocapsules form high loading of CSLNs based on the host molecules and have the ability to act as prospective carrier systems for various guest molecules [16–18]. Contact mode AFM studies have been used to demonstrate the presence of non-aggregated CSLNs (6) in four different gels (xanthan, hyaluronic acid, carbopol 980 and carbopol 2020). The SLNs which are not involved in the surface features can be observed due

Fig. 1. (a) Schematic structures of para-acyl calix[n]arenes (3–6). (b) Non contact mode AFM image of para-dodecanoyl calix[4]arene (6) nanoparticles on mica surface. From Ref. [13], reproduced with permission from Royal Society of Chemistry.

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Fig. 2. Non-contact mode AFM images of: (a) 6-based SLNs dried suspension in pure water; (b) 6-based SLNs reconstituted after freeze-drying in a solution of glucose (2%). From Ref. [20], reproduced with permission from Elsevier Publishers.

to their effects on local surface forces and the local mechanical properties of the gels [19].

2.2. Effect of carbohydrates and ionic strength of salts on CSLNs The effect of cryoprotectant carbohydrate (glucose, fructose, mannose and maltose) on the reconstitution of CSLNs (6) suspensions has been studied by AFM and PCS under freeze-drying conditions. In PCS, no variation of sizes of the 6 based CSLNs were observed even at higher concentration of sugars. The flattened circular objects have a diameter of 55 nm and height of 250 nm whereas the same SLNs after freeze-drying and reconstitution in an aqueous solution of 2% glucose generated spherical objects of 16 nm in height and 270 nm in diameter (Fig. 2) [20]. The stability of CSLNs (6) was investigated by varying the parameters which may occur during the preparation, post-preparation, formulation or treatment and long term storage (Fig. 3), and these were remarkably robust. The effect of freeze-drying/redispersion and the high ionic strengths of certain salts on the stability of the SLNs have also been explored, and under these conditions, the CSLNs can be destabilized [21,22].

2.3. Adsorption of proteins and DNA on the surface of CSLNs The conjugate of calixarene (7) forms stable monomolecular films at the air–water interface and the formation was monitored by Brewster angle microscopy (BAM) at different surface tensions. Addition of tetrahydrofuran in the colloidal aqueous suspension of the calixarene derivative leads to the formation of nanoparticles (as monitored by PCS and AFM studies) and were stable for longer periods (Fig. 4) [23,24]. Polycationic CSLNs of 8 were loaded by a layer by layer (LbL) assembly method and their applications with transfected mammalian cells studied. The adsorption of DNA at the surface of the SLNs was done by incubating them with plasmid DNA at increasing concentrations. The interactions were analyzed by agarose gel electrophoresis and the cell studies have been carried out (Fig. 5) [25]. The CSLNs of 8 form Langmuir monolayers at the air–water interface. The interaction of this molecule with DNA was monitored by looking at the compression isotherm. Such interaction causes an expansion and a slight destabilization of the monolayer. The CSLNs of 8 possesses molecular recognition properties towards DNA (Fig. 6) [26].

Fig. 3. Non-contact mode AFM images of: (a) freshly prepared 6-based SLNs; (b) 1-year-old 6-based SLNs. From Ref. [21], reproduced with permission from Elsevier Publishers.

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Fig. 4. (a) Schematic structure of para-H-tetra-O-dodecyl-calix[4]arene (7). (b) Non-contact mode AFM image of 7-based nanoparticles. From Ref. [23], reproduced with permission from Royal Society of Chemistry.

Fig. 5. (a) Schematic structure of 8 [alk = C12 H23 ]. (b) Confocal micrograph of a transfected cell that has expressed GFP. The red color representing the labeled-chitosan appeared to be in the cytoplasmic compartment of the cell which clearly suggests that 8-SLNs have entered the cell. From Ref. [25], reproduced with permission from Royal Society of Chemistry.

The interaction of CSLNs with biologically relevant molecules such as BSA and DNA was explored by PCS and AFM studies (Fig. 7). The results showed that the albumin acts as capping layer on the surface of the SLNs and even at high concentration of BSA, the SLNs remain non-aggregated and hence can be used

for developing transporters in intra-venous administration. Based on the studies it was proposed that any biomolecule interacting with the CSLNs can either be adsorbed on the SLNs surface or complexed with host molecules or entrapped in SLN matrix [27].

Fig. 6. (a) Schematic representation of monolayers of 8 compressed on a subphase with (below) or without (above) DNA. (b) AFM image of 8-based SLNs spread on mica and imaged in air in non-contact mode. From Ref. [26], reproduced with permission from American Chemical Society.

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Fig. 7. (a) Schematic structures of different amphiphilic calixarenes. Non-contact mode AFM images (from Ref. [27], reproduced with permission from Elsevier Publishers) of 6-based SLNs on mica surface: (b) without BSA and (c) with BSA (20 g/l).

2.4. Small molecular adsorption on the surface of CSLNs

3. Calix[n]arene based gold/silver NPs

129 Xe NMR study of CSLNs of 3, 4, 5, 6, 11 and 12 (Fig. 7) exposed to a constant pressure of 7 Torr of Xe gas showed that the host particle cavities are accessible to small organic molecules and/or small atoms. Such studies also suggested that the complexation with the guest species can be hindered by the adjacent calixarene hydrophobic tail. Self-organization of these CSLNs can be reorganized by flowing guest molecules vapors which can displace the hydrophobic tails from the host cavity and in turn can create new void space [28]. The phosphorylated calixarene derivatives (13, 14) form monolayers at the air–water interface. The interaction of this monolayer with different cations, viz. Na+ , Mg2+ and Ca2+ , was monitored by AFM and DLS studies. All these studies indicated that the divalent metal ions (Ca2+ and Mg2+ ) are capable of inducing flocculation of the SLN colloidal dispersions above certain concentrations. These were further supported by PCS studies (Fig. 8) [29].

Metallic Ag nanocrystals have been made by careful reduction of a four-coordinate Ag complex of ethylenediamine clathrate in calix[4]arene framework (1a) that acts as a reducing agent. The reduction of the Ag complex was confirmed from the color change observed at different temperatures. The color change observed from white to yellow-orange to deep red-brown is resultant of the reflections from [1 1 1] and [2 0 0] planes of the metallic silver [31].

2.5. Toxicity effect of CSLNs The toxicity effect of these SLNs were monitored by the hemolytic effects studied using a series of amphiphilic calixarenes, e.g. 3, 4, 5, 6, 15, 16, on human erythrocytes. The study suggested that neither the chain length of the hydrophobic moiety nor the variation in the nature of the polar head group induces hemolysis in human erythrocytes (Fig. 9) [30].

3.1. Detection of PAHs by AgNPs coated with the conjugates calix[4]arene Dithiocarbamate conjugate of calix[4]arene (17) modified Ag nanoparticles (NPs) was used for sensitive and selective detection of polycyclic aromatic hydrocarbons (PAHs) using surface-enhanced Raman scattering (SERS). The changes observed in the characteristic fingerprint vibrational features in SERS revealed important structural information as well as host–guest interactions (Fig. 10). From the analysis of the spectra delineated that the ␲. . .␲ interaction between 17 and PAHs lead to the formation of charge transfer complex. The detection limit for PAHs {benzo[c]phenanthrene, pyrene and triphenylene} was in the range of 10−7 to 10−8 M [32,33]. The detection of trace amount of PAHs has also been done by SERS using AgNPs covered by adsorbed self-assembled calix[4]arene molecules, viz. 1a, 18 and 19 (Fig. 10). The UV–vis spectra suggests that 18 forms a strong host–guest complex among

Fig. 8. (a) Schematic structures of amphiphilic calix[4]arenas (13 and 14). Non-contact mode AFM images of the solid lipid nanoparticles (from Ref. [29], reproduced with permission from American Chemical Society) formed by: (b) 13 and (c) 14, on mica surface.

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Fig. 9. Schematic structures of the amphiphilic calixarenes studied for their toxicity effects.

the other calixarene derivatives studied, viz. 1a and 19, with PAHs, and finally results in large AgNP aggregates. Comparison of the vibrational spectra of the host–guest complex with that of its parental counterparts, reveal that the interaction is through ␲. . .␲ stacking between the aromatic systems of 18 and the PAHs [34]. The detection of pyrene has also been done by AgNPs of 17 by means of SERS. These NPs were either suspended or immobilized on a glass to exhibit their analytical selectivity towards guest systems bearing four benzene rings, viz. pyrene. Comparison of SERS data at different concentrations was done to find the structural marker bands of the monodentate to bidentate geometries of dithiocarbamate group upon the interaction with the metal and the conformation of the calixarene cavity [35]. PAHs can also be detected as reported in the literature by appropriately modifying the arms of the 1,3-conjugates of calix[4]arene (20), e.g. with naphthylamide moiety which provides a site for the interaction with the PAH as observed in the crystal structure as well as by DFT computations with naphthaldehyde and pyrenaldehyde respectively, as shown in Fig. 11 [36]. 3.2. AgNPs of amino-, phosphonato- and thia-calixarenes and their applications p-Phosphonated calixarenes (21, 22, 23 and 24) have been used as polyphosphonate surfactants and templates for the preparation

of AgNPs of sizes ∼2 nm using hydrogen gas. Control experiments carried out with n-propyl group at the lower rim resulted in no AgNP formation suggesting that the phenolic groups of the calixarene derivatives are essential for the reduction to occur. The pH of the solution is the main factor that affects the particle size and the reaction rate. The studies suggested that the monodispersed particles can be obtained at pH = 12 or after short reaction time with buffered solutions at a lower pH (Fig. 12) [37,38]. Thin films formed by LbL (multilayer) assembled thia-calixrene (25) and tetraamino-calix[4]arene conjugates (26) have been used as nanoreactors for the in situ generation of silver NPs (Fig. 13). The adsorption of silver ions in the films was through the cation. . .␲ and the hydrogen bonding interactions that were extended between the calix[4]arene and the metal ions. The calix[4]arene derivatives (25, 26) act as a reducing agent. The prepared NPs have been characterized by UV–vis spectra, AFM and TEM. The microscopy studies showed that the NPs are highly dispersed and uniform when prepared from 26. The mean size of the particles was <10 nm [39]. Sulfanylalkyl oligo(ethylene glycol) ligands (27) have been successfully used as stabilizers for the MPCs for AuNPs in aqueous medium and additionally have been used for the recognition of immobilized cationic pyridinium moieties by colorimetric and microscopy methods (Fig. 14). These NPs are quite stable and can be re-dispersed in aqueous medium several times without any loss of the materials. The studies showed that even in the presence of 2 M

Fig. 10. (a) Schematic structures of 17, 18 and 19. (b) Scheme showing the complexation mechanism with the formation of the highly sensitive inter-particle junction using NPs of 17.

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Fig. 11. (a) Schematic structure of 20. (b) The crystal structure of 20 co-crystallized with salicylaldehyde. (c) Molecular dynamics (MD) simulated structure of 20 with pyrenaldehyde. From Ref. [36], reproduced with permission from American Chemical Society.

NaCl solution the particles do not aggregate. The size of the particles were in the range of (14 ± 1 nm) as confirmed by TEM. The conjugate 27 retains its cation binding ability in aqueous medium even when immobilized in the ligand shell of the MPCs. This was confirmed by the studies carried out with the beads of molecular sieve primed with pyridinium ions of (4-methylphenyl)sulfonyl derivative or Au surface coated with the same [40]. Calix[4]arene derivatives bearing two alkanethiol chains of variable lengths (28, 29) have been used for AuNP preparation by MPC method. The recognition ability of these AuNPs was checked by their association with the cations of N-methylpyridinium tosylate by 1 H NMR titration in CDCl3 . The studies suggested that the recognition ability of AuNPs depends on (a) number of spacer CH2 groups present between AuNP surface and the calixarene derivative, where the recognition is better when the number is more, and (b) the number of calixarene units present on the AuNP surface (Fig. 15) [41].

Similarly a series of calixarene derivatives, viz. 30, 31 and 32, have been prepared and the effect of the denticity of the conjugates on the size of the AuNPs was reported. Apart from using three different derivatives, these AuNPs have been synthesized at three different calixarene conjugate to Au ratios. The studies suggest that the size of the AuNPs prepared from the monodentate derivative (30) is always higher as compared to the AuNPs prepared from its bidentate (31) or tridentate counterparts (32). By using the 31 and 32, the AuNPs of ∼1 nm could be prepared (Fig. 16) [42]. A library of thioether conjugates have been used for the preparation of AuNPs and among these only tetrakis(thioether) derivative of calixarene, 33, gave the smallest particle size and narrowest size distribution. The AuNPs were characterized by 1 H NMR and TEM studies. The better stability of AuNPs observed in the presence of 33 was attributed to the possible role played by the orientation of the alkyl chains in the calixarene which arranges in a densely packed fashion (Fig. 17) [43].

Fig. 12. (a) Schematic structural formula of p-phosphonated calix[4]arene. (b) TEM of silver nanoparticles using 0.25 mM of phosphonated calix[4]arenes. Inset: histogram of particle size from measuring 1157 particles. (c) HR-TEM of a ∼2.3 nm silver particle. Inset: Fourier transform of particle down 1 1 0 zone axis for face-centred cubic silver. From Ref. [37], reproduced with permission from Royal Society of Chemistry.

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Fig. 13. (a) Schematic structures of 25 and 26. (b) TEM images of (26/AgNP)3 (from Ref. [39], reproduced with permission from Elsevier). (c) Scheme for the self-assembly process of the (calix[4]arene/AgNPs) films.

A thiolate-calix[4]arene host, 34, was used for AuNPs preparation through an exchange reaction in toluene starting from tetraoctyl ammonium bromide (TOABr) stabilized gold nanoparticles (Fig. 18) of ∼6 nm size. The self-assembly of these NPs in the presence of dialkyl bipyridinium-based guest molecules have been monitored by absorption, TEM and DLS studies and showed that the aggregation occurs through supramolecular interactions present between the host and the guest moieties, and the size and the solubility of these aggregates can be controlled by the length and the rigidity of the guest molecule [44]. Structurally tailored calixarene conjugates (35, 36 and 37) have been used to coat AuNPs. Such an assembly was used to tune the guest access to the calixarene cone cavity for the cationic recognition (Fig. 19). The NPs have been characterized by TEM, absorption, Raman and FTIR studies. The cation recognition studies

have been performed by spectrophotometric and colorimetric methods. Among the cations studied, viz. K+ , Cs+ , Ba2+ , Pb2+ , Cu2+ , Ca2+ , only Cs+ showed largest red shift. The change in the SP band was attributed to the aggregation of the nanoparticle in the presence of cations. A possible mechanism for such ionic recognition by these nanoprobes has also been proposed [45]. Correlation between the accessibility and the ligand flexibility in the bound state of the AgNP surface was drawn by using a family of phosphine-calix[8]arene conjugates, 38, 39 and 40 (Fig. 20). The results confirmed that the rigid molecules have no access, whereas flexible ligands lead to accessible surfaces when bound to the same AuNPs. These results are consistent with an induced-fit mechanism and suggest that a flexible bound ligand changes subtly in shape in order to accommodate adsorption of an incoming molecule on the metal surface [46,47].

Fig. 14. (a) Reaction scheme illustrating the one-step stabilization and functionalization of gold NPs with (1-sulfanylundec-11-yl) tetraethylene glycol and 27 carried out in a THF/water mixture. (b) TEM image of the 14-nm gold NPs, that are stabilized and functionalized with (1-sulfanylundec-11-yl) tetraethylene glycol and 27. From Ref. [40], reproduced with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Fig. 15. (a) Schematic structures of 28 and 29. (b) Schematic representation of gold MPCs functionalized with calix[4]arenes.

SH HS HS HS

HS

9

OH O OH OH

OH O O HO

30

9

9

9

9

OMe O

31

HS

O

MeO

9 OMe

O

32

Fig. 16. Schematic structures of 30, 31 and 32.

Fig. 17. (a) Schematic structure of calixarene thiother derivative used for the preparation of gold colloids. (b) TEM images of 33 protected gold colloids. (c) Corresponding histogram showing the size distribution of the particles seen in (b). From Ref. [43], reproduced with permission from Royal Society of Chemistry.

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Fig. 18. (a) Schematic structure of the 34 used for the stabilization of the AuNPs and of the monofunctional and bifunctional pyridinium-based organic salts used for the networking of the nanoparticles (G1–G4). (b) Schematic representation of the synthesis of the 34-AuNPs prepared from NP(34) by using tetraoctyl ammonium bromide (TOABr) as the stabilizer.

Fig. 19. Illustrations of the functionalization of Au nanoparticles by calixarene conjugates with different number of methylthio groups (35, 36 and 37).

3.3. NPs of the conjugates of calixarenes by direct synthesis and/or post-synthetic surface modification The interactions between the ␲-electron rich calixarene cavities and the surface of AuNPs have been demonstrated by using different mercaptocalixarene lignds (41, 42) for the preparation of AuNPs by adapting two different synthetic methodologies, viz.

post-synthetic surface modification or direct synthesis approach (Fig. 21). The results obtained were compared with an analog monomer derivative (43). Irrespective of the synthetic approach used, 41 exhibits max in SPR band higher than the other two and shows a shift of ∼5 nm from its original position of 300 nm and this is followed by a ∼10% increase in the absorption intensity. This is possibly because of the orientation of its calixarene cavity towards

Fig. 20. Schematic structures of calix[8]arene based phosphine derivatives.

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Fig. 21. Schematic structures of 41, 42 and 43.

Fig. 22. Schematic structures of 44 and 45.

the gold surface upon adsorption. No change in either SPR band or in the absorbance was observed for 42 and 43 since in case of 42 the cavity points away from the surface and in case of 43 it lacks the calixarene cavity [48]. Post-synthetically surface modified AuNPs have been prepared from the chiral calixarene ligands (44, 45) which exhibit a CD-active SPR band (Fig. 22). This was confirmed by almost 10 fold increase observed in the ligand molar ellipticity when bound to the Au surface as compared to that when the ligands are free in solution. The origin of the band was referred to the asymmetry in the electronic structure of the metal NP core which has direct correlation with the nearby asymmetric center of the adsorbed ligand [49].

spherical Au and Ag NP assemblies (Fig. 23). To achieve this in solution, thiol-functionalized barbituric acid 5-ethyl-5(10mercaptodecyl) barbiturate (EMDB) was used as one of the double rosette building blocks. The NP assembly was monitored by 1 H NMR, TEM, DLS and UV–vis spectral studies. The morphologies of the NP assemblies can be tuned by changing the amount of the building block that chemisorbed on the NP surface. The large degree of NP agglomeration in the presence of 46 was attributed to the higher extent of double rosette mediator formation on the nanoparticle surfaces [50]. Similarly 46 and 47, together with 5,5-diethyl barbituric acid (DEB) or para-cyano-phenyl cyanuric acid (CNPhCYA) have been used as double rosette assemblies for the preparation of the silver complex (Fig. 24). Such Ag complex was subjected to an electron beam at 200 keV for the in situ generation of AgNPs on a TEM grid. Using this methodology, monodispersed AgNPs of very small sizes (∼2 nm) have been obtained. The formation of Ag(0) particles have been further characterized by HRTEM and EDX analysis [51]. 4. para-Sulfonated calixarene for NP/SWCNT preparations para-Sulfonato-calixarenes (48) can be synthesized either by direct ipso-sulfonation of 1a or by a direct sulfonation or chlorosulfonation of 2a (Scheme 2). Because of the advantage of being water soluble, these have been widely used for the detection of various biologically important molecules [52].

3.4. Double rosette assemblies of NPs

4.1. Magnetic nanoparticles stabilized by normal and sulfonated conjugates of calixarenes

Multicomponent (nine building box) double rosette molecular boxes of calixarenes (46) have been used as mediators for

The surface of magnetic nanoparticles (MNPs) have been stabilized and modified in a convenient in situ process by using

Fig. 23. (a) Schematic structure of 46. TEM images (from Ref. [50], reproduced with permission from American Chemical Society): (b) EMDB protected AuNPs; (c) EMDB protected AuNPs when stirred with 46.

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Fig. 24. (a) Schematic structure of 47. HRTEM analysis (from Ref. [51], reproduced with permission from Royal Society of Chemistry RSC for Centre National de la Recherche Scientifique and the RSC) of AgNPs formed in the presence of: (b) Ag+ [463 (DEB)6 ] and (c) Ag+ [473 (CNPhCYA)6 ] assemblies. Scale bars 5 nm.

-O S 3

-O S 3

SO3-

Ipso-sulfonation H 2SO 4, 800 C OH

OH OH

HO

OH

OH OH

1a

SO 3Direct sulfonation H2SO4, 1000 C Or HSO 3Cl, CH 2Cl2 HO Chloro-sulfonation

48

OH

OH OH

HO

2a

Scheme 2. General synthetic routes to prepare p-sulfonato calixarenes.

p-sulfonato-calix[6,8]arene (49 and 50) (Fig. 25). The formation of the NPs was confirmed by TEM and their magnetic properties have been measured by SQUID. The nature of the interactions of the MNPs with that of the calixarene surfactants have been established by DLS and FTIR studies. These MNPs were stable at physiological pH. At room temperature, these exhibit superparamagnetic behavior with high saturation in magnetic moments [53]. Calix[6]arene (1c) and its sulfonated form (49) have been used for the deposition on iron oxides and the resultant MNPs were obtained as nanocolloidal solution (Fig. 26). The results obtained from the TGA, DTA and FTIR clearly show strong bonding between the macrocyclic molecules and the nanoparticles. This result is also being supported by the fact that in the TEM, only very few particles were in the aggregated state. Comparison of the stretching frequencies observed in FTIR of the two compounds in conjunction with the thermal studies suggests that while it is the OH group that is attached in case of calix[6]arene with Fe2+ /Fe3+ , it is the SO3 H moiety in case of the sulfonated ones [54]. The negatively charged SO3 groups at the upper rim electrostatically repel other nanoparticles and hence prohibit nanoparticle agglomeration. These MNPs have been further used to dictate the self-assembly of doped LaPO4 nanorods into three dimensional “koosh nanoballs”

like morphology as a bifunctional luminescent ferro-fluidic system (Fig. 27) [55]. 4.2. SWCNTs protected by the water soluble sulfonated and phosphonated calixarenes p-Sulfonated calixarenes (48, 49 and 50) act as protecting agents on the surface of SWCNTs and thus inhibit their aggregation due to electrostatic repulsions. The solubility of the SWCNTs in aqueous medium is much higher in the presence of 50 as compared to 48, 49 and p-phenolsulfonic acid and hence 50 acts as a better protecting agent. Upon the addition of the guest dimer (and not the monomer), the formation of polymeric network of SWCNTs was observed as these species goes to the vacant side of 50 and this is being supported by the DLS studies (Fig. 28) [56]. Water soluble p-phosphonated calix[n]arenes (51–57) and their upper rim sulfonated versions (58 and 59) have been used in an effort to solubilize SWCNTs into aqueous medium. The calixarene derivatives with larger ring size (hence conformationally flexible) possessing larger hydrophobic surface area involve in the ␲. . .␲ stacking with the surface of SWCNTs and thereby enhances the water solubility. The strong binding affinity of sulfonato calixarene

Fig. 25. (a) Schematic structure of p-sulfonato-calix[n]arenas. (b) Possible mode of interaction of 49 at the surface of the nanoparticles (NPs). (c) TEM micrograph (from Ref. [53], reproduced with permission from Royal Society of Chemistry) shows the coating of 49 on the surface of the magnetite nanoparticles.

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Fig. 26. TEM micrographs of (a) 1c coated on amorphous Fe2 O3 colloidal particles and (b) 49 coated on amorphous Fe3 O4 colloidal particles. From Ref. [54], reproduced with permission from American Chemical Society.

Fig. 27. (a) A schematic representation of the Koosh nanoball structure of LnPO4 nanorods (Ln = La, Eu, Tb, Ce) held together by 49 stabilized Fe3 O4 nanoparticles. (b) TEM image and high-resolution TEM image of (1 and 2) Fe3 O4 @49–LaPO4 :Eu3+ and (3 and 4) Fe3 O4 @49–LaPO4 :Ce3+ :Tb3+ nanocomposites (insets to 2 and 3 are the corresponding FFT and SAED patterns). From Ref. [55], reproduced with permission from Royal Society of Chemistry.

Fig. 28. (a) Aqueous media of SWCNTs with (1) 48, (2) 49 and (3) 50, after sonication. Micrographs of water-soluble 50/SWCNT hybrids: (b) TM-AFM and (c) TEM. From Ref. [56], reproduced with permission from Royal Society of Chemistry.

Fig. 29. (a) Calixarene structure and notation for calix[n]arene systems. (b) TEM image (from Ref. [57], reproduced with permission from Royal Society of Chemistry) illustrating SWCNT networks coated with 53.

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Fig. 30. (a) Photographic images of 48 protected AgNPs solutions containing different amino acids. TEM images of 48 protected AgNPs (1) before and (2) after addition of 10−4 M His. Both the scale bars are 50 nm. From Ref. [59], reproduced with permission from Royal Society of Chemistry.

towards the SWCNTs has also been deduced based on Raman spectroscopy. The topology and electronic nature of the selectively solubilized SWCNTs have been determined (Fig. 29) [57]. Similar studies have also been carried out with p-sulfonato-calix[n]arenes, where n = 4, 6, modified by butyl and dodecyl substituents [58]. 4.3. AgNPs of the water soluble conjugates of calixarenes and their applications Water soluble calix[4]arene conjugate (48) has also been used as monolayer protecting cluster for AgNPs and found that these are better than their non-cyclic counter parts owing to their macrocyclic effect and yields ∼8 ± 1 nm AgNPs. These NPs recognize histidine colorimetrically among the amino acids studied and the color change was attributed to a concomitant change in the NP aggregation as delineated by TEM (Fig. 30) [59]. Similarly 48 protected AgNPs have been used for the detection of pesticides and the aggregation of NPs have been observed by TEM [60]. Similarly 48 coated AgNPs have also been used for the colorimetric and spectroscopic discrimination of nucleotides vs. nucleosides [61]. p-Sulfonatocalix[6]arene (49) modified AgNPs have been used as coating agent on a glassy carbon electrode (49-AgNPs/GCE). Studies showed that 49 is highly efficient to capture organophosphates (OPs). This also increases the enrichment of nitroaromatic OPs onto the electrochemical sensor surface such as methyl parathion. The simultaneous determination of methyl parathion was carried out by the differential pulse voltammetry (DPV) [62]. The oleic acid protected AgNPs in hexane (hydrophobic layer) have been transferred into the hydrophilic layer by stirring this with the sulfonatocalixarene derivative (48). By mixing the two solvents, the hydrophobic tail of the oleic acid enters into the

hydrophobic arene cavity and thus forms an inclusion complex. Such phase transfer ability of AgNPs to aqueous solution depends on the initial concentration of 48. The phase transfer was monitored by UV–vis, TEM, FTIR and 1 H NMR. This brings a change in the surface property of the silver nanoparticles from hydrophobic to hydrophilic and thus expected to have applications in the area of biology, catalysis and sensors (Fig. 31) [63]. Such studies have also been extended to the biphasic mixture of alkanethiol in chloroform and aqueous solution containing p-sulfonatocalixarene derivative (48) [64]. Titrations of 60-AgNPs with different anions were performed, of which, only HSO3 − exhibit color change from orange-yellow to orange-red immediately. The other oxo-anions, such as, HSO4 − and SO4 2− , do not show any color change. In TEM, 60-AgNPs were seen as spherical particles and mostly monodispersed with sizes of ∼5–8 nm (88%). When HSO3 − was added to 60-AgNPs, the size of the particles increases due to the aggregation and results in nonspherical ones. The aggregation was modeled by computation to give the structure shown in Fig. 32 [65]. 4.4. AuNPs of the water soluble conjugates of calixarenes and their applications p-Sulfonatocalix[6]arene (49) modified AuNPs have been synthesized in aqueous medium and were used as probes for the colorimetric detection of the isomers of diaminobenzene (DAB). The color changes have been attributed to the aggregation of NPs where DAB is involved in bridging the nanoparticles via electrostatic interaction and host–guest interaction (Fig. 33) [66]. p-Sulfonatocalix[4]arene-thiol modified (61) AuNPs have been used as colorimetric probe to sense lysine, arginine and histidine in water among the amino acids studied. The aggregation of NPs in

Fig. 31. (a) TEM micrograph of the silver nanoparticles (1) in hexane before phase transfer, (2) particle size distribution histogram of (1). (b) TEM micrograph of the silver nanoparticles (3) in 48 aqueous solution after phase transfer, (4) Particle size distribution histogram of (3). From Ref. [63], reproduced with permission from American Chemical Society.

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Fig. 32. Schematic structure of (a) 60. TEM of (b) 60-AgNP and (c) [60-AgNP + HSO3 − ]. Insets: photographic images of 60-AgNP and [60-AgNP + HSO3 − ]. (d) Schematic representation of the aggregation of AgNPs (from ref. [65]).

Fig. 33. (a) Photographic images of 49 protected AuNPs with diaminobenzenes (DABs) isomer (1) o-DAB, (2) m-DAB, (3) p-DAB and (4) control. (b) TEM images of 49 protected AuNPs after the addition of (5) o-DAB, (6) m-DAB and (7) p-DAB. All scale bars are 100 nm. From Ref. [66], reproduced with permission from Elsevier Publishers.

the presence of these amino acids was supported by TEM (Fig. 34) [67]. Water soluble p-sulfonato calixarenes have been used for the preparation of NPs of trans-␤-carotene using spinning disk processing (SDP) method. The effect of reaction conditions, such as, the choice of surfactants, macrocyclic amphiphiles, the calixarene cavity size, and ␣, ␤-cyclodextrins, on the nanoparticle formation was reported. The mean diameter of the carotene nanoparticles vary depending upon the simple sulfonato-calix[4,5, 6 and 8]arene (48, 62, 49 and 50) used, where the studies were supported by the dynamic light scattering (DLS) [68].

4.5. Cu2 ONPs of the water soluble conjugates of calixarenes and their applications Cuprous oxide nanoparticles have been prepared in aqueous medium containing p-sulfonated calix[8]arene (50) as receptors and hydrazine as reducing agent. The nanoparticles were characterized by powder XRD, TEM, SEM, FTIR, UV–vis spectra, EDX and thermogravimetry. The nanoparticle formation is strictly concentration dependent of both 50 and CuSO4 as observed from TEM.

At lower concentration of CuSO4 , almost uniform nanoparticles of ∼10 nm were found whereas at higher concentration of CuSO4 , different architectures, viz. nanospheres (∼100–200 nm), nanorings or hollow nanospheres were formed, where 50 possibly plays a bridging role between the nanoparticles (Fig. 35) [69].

5. Calix[n]arene QDs and other NPs 5.1. QDs Biocompatible, highly fluorescent and stable CdSe–ZnS QDs were prepared by using carboxylic acid derivatives of calix[4]arene (Fig. 36) (63) as surface coating agents in aqueous medium. The QDs coated with 63 exhibit much higher emission efficiency as compared to the conventional coating agent, viz. mercaptopropionic acid (MPA) or mercaptoundecanoic acid (MUA) by a factor of 3.5–20. The sizes of these QDs were <10 nm. The calixarenes exhibit hydrophobic interaction with the trioctylphosphine oxide (TOPO) capped QDs. Comparison of the sizes of the QDs prepared from different coating agents, reveals that 63 forms a bilayer structure with TOPO that stabilizes the surface of QDs [70].

Fig. 34. (a) Schematic structure of 61. (b) Photographic images of calix-capped gold nanoparticles solutions containing different amino acids. From Ref. [67], reproduced with permission from Royal Society of Chemistry.

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Fig. 35. TEM images (from Ref. [69], reproduced with permission from IOP Publishers) of 50 modified Cu2 O nanoparticles. (a) c50 , 0, cCuSO4 , 5 × 10−3 mol l−1 ; (b) c50 , 1 × 10−3 mol l−1 , cCuSO4 , 5 × 10−3 mol l−1 ; (c) c50 , 1 × 10−3 mol l−1 , cCuSO4 , 2 × 10−2 mol l−1 ; (d) c50 , 5 × 10−4 mol l−1 , cCuSO4 , 2 × 10−2 mol l−1 ; (e) c50 , 1 × 10−4 mol l−1 , cCuSO4 , 2 × 10−2 mol l−1 .

5.1.1. QDs of sulfonated calix[4]arene Water soluble amphiphilic p-sulfonatocalix[4]arene derivatives (48, 64 and 65) have been used for the surface modification of CdSe/ZnS QDs. These QDs were monodispersed. By using the diffusion time in FCS experiments, the size of these QDs was calculated to be 12 nm. The higher emission efficiency of QDs protected by 65 as compared to the mercaptoacetic acid (MAA) protected ones was attributed to a higher barrier towards the access of water molecules onto the QD surface in the former. The QDs protected by 65 were used for the optical detection of the neurotransmitter acetylcholine (ACh) which was monitored by the significant quenching of the fluorescence of these QDs. It was proposed that these calixarene derivatives, act as host molecules towards ACh which act at the water–QD interface and hence the recognition occurs (Fig. 37) [71]. Highly fluorescent, stable and water soluble QDs of CdSe were synthesized using p-sulfonatocalixarene derivatives (48 and 49) by ligand exchange method from TOPO coated ones. The QDs coated with 48 showed sensitivity towards methionine by exhibiting an increase of ∼82% in the emission intensity, whereas 49 coated ones have high affinity towards phenylalanine with a 61% increase in the intensity in the physiological buffer solution among all the amino acids studied [72].

5.1.2. Host–guest complexes of QDs and core shell QDs Luminescent CdSe–ZnS core–shell quantum dots were prepared using either TOPO or calixarene conjugate (66). The size of the QDs was estimated using absorption data and stoichiometric considerations as well as DLS studies. The QDs coated with 66 have higher sizes as compared to its TOPO counterpart, possibly because of the larger size of 66 and the existence of its higher aggregate nanoparticles. The binding of different bipyridinium dications (G5, G6) with these QDs was monitored by spectrophotometry as well as by fluorescence quenching. The association constants of the resulting host–guest complexes were in the range of 104 –107 M−1 in CHCl3 . Interestingly, in the presence of 67 the bipyridinium quenchers desorb from the surface of QD–TOPO and associate with 67 and results

O CO 2H

O

O

CO2 H HO 2C

O HO2C

Calix[4]tetracarboxylic acid = 63 Calix[6]tetracarboxylic acid = 68 Calix[8]tetracarboxylic acid = 69 Fig. 36. Schematic structures of 63, 68 and 69.

in the restoration of the original luminescence intensity of the QDs (Fig. 38) [73]. The fluorescence response of the gemini surfactant (GS), viz. [(C12 H25 )(CH3 )2 N+ (CH2 )4 N+ (CH3 )2 (C12 H25 )]·2Br− modified QDs of CdSe towards Tyr and Cys is altered in the presence of psulfonatocalix[4]arene (48). In the presence of 48, the GS coated QDs show significantly high luminescence emission for only Cys as compared the control where 48 is absent. In both the cases, except for the amino acids concerned, only Ser and Ala shows changes in the intensity among all the other amino acids studied. The possible mechanism of such recognition is schematically shown in Fig. 39 [74]. The surface modified calix[n]arene (63, 68, and 69) coated QDs were used to control the optical properties synthesized from TOPO-coated QDs of CdSe (Fig. 36). Such coating with calixarene derivatives exhibits red shift in the emission peak of the QDs. This shift increases with increasing oligomeric size of the calix[n]arene used. The calculated hydrodynamic diameter for all the three calixarene coated QDs is ∼10 nm [75]. A selenium derivative of calixarene (70) was used for the preparation of core/shell QDs of CdSe/ZnS. The fluorescence intensity of the 70 capped QDs increases about 110% (and the quantum yield ∼45%) compared to the original core/shell QDs of CdSe/ZnS after five hours. The QDs were characterized by TEM and PCS studies. In both the studies that the size of the QDs increase after coating with 70 and the coating was confirmed based on the solution state 1 H NMR studies (Fig. 40) [76]. In a similar fashion the luminescent and stable core/shell QDs of CdSe/ZnS with capping by a thia-calixarene (71) are prepared. These QDs were monodispersed with almost uniform size as studied based on TEM. The 71 protected QDs were successfully used for highly sensitive and selective optical recognition and determination of mercury ions in acetonitrile based on the host–guest interaction on calixarene-based surface architectures. In presence of Hg2+ , the fluorescence intensity of the QDs is quenched in a concentration dependent manner (Fig. 41) [77]. 5.1.3. Detection of pesticide and PAH by QDs Highly luminescent and stable QDs of CdTe in sol–gel-derived composite silica spheres coated with calixarene derivative (72) were prepared in aqueous medium (72/SiO2 /CdTe). When coated with 65 or any other higher oligomers of calixarene (1c and 1d), these NPs showed higher fluorescence intensity and quantum yields as compared to the simple NPs of SiO2 /CdTe. The fact that the fluorescence enhancement is low in case of simple p-tertbutylphenol, a monomeric version of calixarene coated NPs implies that the calixarene cavity with electron-rich benzene-␲ system and the hydrophobic wall plays important roles in protecting the surfaces of nanoparticles. These coated NPs were used for the optical recognition of methomyl, a pesticide. Among all the other pesticides used, the presence of methomyl brings an increase the fluorescence intensity of the NPs as a function of the concentration

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SO3- SO3 -

-O S - O3 S 3

SO3- SO3 -

-O S - O3 S 3

SO3 -

SO 3- SO3 -

O O O

O

P O

P O

P

O

O

O

O R

O

O

R

O

P

O

65

P

P

O

P

R

R

P

48 64 65

O

MAA = Mercaptoacetic acid

O

A MA

R=H R = CH 3 R = (CH 2) 5CH 3

O

SO3SO 3SO3SO3O O O O

O

O O O

-O S 3

2113

TOPO capped QDs

O

COOH

O

S OSO33SO3-

SO3 S

COOH

S

O O O

O

S

COOH

SO3- SO3

-O S -O S 3 3

(a)

-

(b)

Fig. 37. (a) Molecular structure of p-sulfonatocalix[4]arene derivatives. (b) A schematic representation of the surface-coating of TOPO capped CdSe/ZnS QDs with 65 and MAA.

which can be effectively described by a Langmuir-type binding isotherm (Fig. 42) [78]. Similarly, p-sulfonatocalix[4]arene (48) modified QDs of CdTe were used to detect pesticides in aqueous solution [79]. Highly luminescent and stable CdTe nanocrystals were prepared via a sol–gel technique in aqueous medium by using calixarene

(72 and 1d) coated silica nanospheres. These CdTe NPs were highly sensitive towards PAHs, viz. anthracene and pyrene [80]. A calixarene derivative, 73, forms highly anisotropic nanofilaments of uniform width as studied by TEM. When Cd(II) reacts with S2− in the presence of 73, individual cylinders of 73 were covered with monodispersed CdS nanoparticles of almost 2 nm size

HN O

R1

N+

N+

R1

- R1 G52+ - (CH2)9Me G62+ - (CH2)7Me

NH O

NH

O R

2

O O Me Me

HN

O

HN NH

O

O R2

O R2 Me

- R2 66 - (CH2)11SH 67 - (CH2)7Me

Fig. 38. Schematic structure of the bipyridinium dications G52+ and G62+ and the calixarenes 66 and 67.

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Fig. 39. A schematic illustration of a possible mechanism of the fluorescence enhancement of GS-QDs by (a) Tyr and (b) Cys.

Fig. 40. (a) Schematic structure of 70. (b) TEM micrographs (from Ref. [76], reproduced with permission from Elsevier Publishers) of (1) original QDs, scale bar is 50 nm and (2) QDs capped with 70, scale bar is 100 nm.

Fig. 41. (a) Schematic structure of 71: (b) TEM images (from Ref. [77], reproduced with permission from Elsevier Publishers) of (1) original QDs and (2) QDs capped with 71.

Fig. 42. (a) Schematic representation of the formation of 72/SiO2 /CdTe NPs. (b) TEM images (from Ref. [78], reproduced with permission from American Chemical Society) of (1) CdTe QDs and (2) 72/SiO2 /CdTe NPs.

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Fig. 43. (a) Schematic structure of an amphiphilic dendro-calixarene 73. (b) TEM micrographs (from Ref. [81], reproduced with permission from Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) of (1) unstained two discrete CdS-decorated cylindrical micelles of 73 showing spatially organized electron-dense quantum dots. The inset shows the corresponding EDX analysis with Cd and S peaks. (2) High-magnification image showing surface patterning of CdS nanoparticles along an individual micelle; scale bar = 40 nm. The inset shows a high-resolution TEM image of an individual CdS nanocrystal showing a 2D lattice image with two sets of {1 1 1} lattice fringes (0.33 nm) associated with the zinc blende structure; scale bar = 1.3 nm.

as observed from HRTEM. These QDs of CdS were found at discrete surface sites and they were spatially separated along the supramolecular structure. The presence of Cd and S was confirmed by energy dispersion X-ray analysis (EDX) (Fig. 43) [81].

5.2. Other NPs 5.2.1. TiO2 nanoparticles NPs of TiO2 coated with monolayer of calix[4]arene glycoclusters (74 and 75), have been prepared. The coating was carried out by using the click reaction between azido-functionalized calix[4]arene-coated nanoparticles with free hydroxy propargyl carbohydrate derivatives. The completion of the reaction was monitored by observing the disappearance of the azide peak of calixarene derivative by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. This method resulted in the immobilization of calix[4]arene-based glycoclusters on the TiO2 surface. Upon stirring these NPs in 1:1 of CH3 OH–H2 O mixture at room temperature overnight, no release of calixarene was observed. Hence the coating on the NP surface is quite stable. Such materials can be used for the study of multivalent carbohydrate–protein interactions using nanoscale properties (Fig. 44) [82]. Calixarene and thia-calixarene derivatives (76–80) have been attached covalently and electronically onto the surfaces of TiO2 nanoparticles. These surface grafted calixarene derivatives were hydrolytically stable. Such stability was referred to originate from the multiple covalent connections, possible between the calixarene derivatives and Ti surface centers. The covalent interactions between the calixarene derivatives and TiO2 surface were confirmed from the appearance of a LMCT absorption peak in diffuse reflectance UV–vis spectroscopy. The steady state photoluminescence emission of the calixarene–TiO2 hybrid material decreases with increasing the electron withdrawing ability of the grafted calixarene derivatives (Fig. 45) [83].

5.2.2. Magnetic nanoparticles Using the aminolysis reaction, the diester derivative of calix[4]arene (81) was immobilized onto the surface of 3aminopropyltrimethoxy silane (APTMS) modified-Fe3 O4 nanoparticles. The synthesized nanoparticles were characterized by IR, TGA, elemental analysis and TEM studies. After grafting of 81 onto the nanoparticle surface, the dispersion of the particles was improved significantly even when compared to the simple APTMS modified particles. This is possibly because of the electrostatic repulsion and the steric hindrance between 81 and the surface of Fe3 O4 nanoparticles. The extraction studies carried out with HCr2 O7 − indicated that the calixarene modified magnetic nanoparticles are better receptors for this anion as compared to the silane modified nanoparticles at pH 2.5–4.5. The counter cation, Na(I), plays important role in such ion-pair extraction (Fig. 46) [84,85]. Similarly, some other calixarene derivatives have also been used as grafting agents for nanoparticle preparation and the anion recognition studies were carried out with those MNPs [86]. These MNPs provide chemically inert sol–gel support to encapsulate Candida rugosa lipase (CRL). Such MNPs of calix[4]arene (82) encapsulated lipase have excellent enantioselectivity as compared to the free enzyme (Fig. 47) [87]. Two different calixarene di-amide derivatives attached with pyridyl moiety (83 and 84) were synthesized and characterized by various analytical techniques. These derivatives were immobilized onto the surface of [3-(2,3-epoxypropoxy)-propyl]trimethoxysilane (EPPTMS)-modified Fe3 O4 MNPs to result in calixarene coated magnetic nanoparticles. Such magnetic nanoparticles were characterized by a combination of elemental analysis, FTIR, TEM and thermogravimetric analysis (TGA). The extraction ability of these MNPs towards different uranyl ions [U(VI)] at various pH was compared with simple calixarene derivatives (83 and 84) but not the nanoparticles. The extraction ability of the calixarene coated MNPs are significantly higher than their monomeric precursors. With these MNPs, the removal of U(VI) is maximum at

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Fig. 44. (a) Schematic structures of calix[4]arene based glyco-derivatives on TiO2 surface. (b) SEM image (from Ref. [82], reproduced with permission from Royal Society of Chemistry) of glyconanoparticles of 74.

pH 5.5–8.0. Different models for the enhanced extraction ability of these MNPs have also been proposed (Fig. 48) [88]. Similar type of pyridine based conjugates of calixarene (85 and 86) grafted MNPs have also been studied for their extraction of toxic oxyanions, such as, arsenate and dichromate from aqueous

R1

R1

OH

O

R1

R1

X

X

X

solution [89] (Fig. 49). These are from aminopyridyls rather than from amino picolyls present in case of 83 and 84. In a similar way, 87 based MNPs have excellent affinity and selectivity towards oxo-species of Cr(VI), As(V) and U(VI), even in the solution containing a great deal of competing foreign ions [90].

O

O

O

O

R2

Ti

X

X

X

X

O Ti

O

O

Ti

M

TiO 2

MO2 = TiO2 /SiO2

R1 = R1 = R1 = R1 = R2 =

C4H9, Br, NO2, C4H9, CH 3,

X = CH 2 X = CH 2 X = CH2 X=S X=S

76 77 78 79 80

Fig. 45. Schematic structure of calixarene derivatives used for the studies.

O

X

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Fig. 46. (a) Surface modification of 81-grafted magnetic nanoparticles. (b) TEM micrographs of calix[4]arene-grafted Fe3 O4 (from ref. 84, Reproduced by permission of Elsevier Publishers). From Ref. [84], reproduced with permission from Elsevier Publishers.

MNPs coated with calixarene based 4-amino-1-benzylpiperidine conjugates (88 and 89) were studied for the removal of As(V), Cr(VI) and U(VI) ions from aqueous solutions in liquid–liquid/solid–liquid extraction experiments (Fig. 50) [91]. Calixarene-crown-6 derivatives with terminal acyl fluoride (90) were synthesized and attached to nano-sized magnetoferritin molecules. These chelate attached magnetoferritin NPs have been used for their ability to sequester radioactive Cs(I) ions from aqueous solution. The advantage of using these nanoparticles as

OH

OH

HO

OH

HO

OH HO

OH

OH

HO

sequesters is attributable to their smaller size compared to the conventional ion exchange beads and their ion specific nature due to the functionalization. Comparison of the functionalized nanoparticles with those of non-functionalized ones have been carried out to prove that the former has much higher affinity towards Cs(I). This result also suggests that 90 does not lose its affinity towards Cs(I) even when attached to NPs. The paramagnetic NPs chelated with the radioactive Cs(I) ion can be removed by magnetic filtration and can be further concentrated by evaporation (Fig. 51) [92]. Calixarene derivative functionalized with thioester group (91) was used for the synthesis of Pd, Pt and Ru nanoparticles from their corresponding zerovalent metallic complexes. The crystalline nature of all the three NPs was confirmed by electron diffraction studies. In case of Pd and Pt, very small monodispersed nanoparticles were obtained (diameter ≤ 1 nm), whereas for ruthenium, aggregated NPs of 2–3 nm were obtained. These NPs have high chemical stability in the acidic medium. The inclusion of CO in the calixarene cavity of 91 that is present on the surface of Pt and Ru NPs has been confirmed from IR spectral studies (Fig. 52) [93].

N

N

6. Guest species (metal ion/amino acid/protein) induced microscopy

OH

OH OH

HO

82 Fig. 47. Schematic structure of 82.

The use of weak interaction as well as the coordination bonds led to the construction of sophisticated supramolecular architecture or materials in a range of different dimensions. Here, in this part, attention was focused at understanding the differences in the structural features of the species at the microscopic level (using

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Fig. 48. (a) Schematic structure of 83 and 84. TEM micrographs (from Ref. [88], reproduced with permission from Elsevier Publishers) of (b) pure Fe3 O4 nanoparticles and (c) calixarene coated magnetic nanoparticles.

O

O

O

O

OH OH

O

O

O

O

O

O

O

O

O

O NH

HN

OH OH

NH

HN

N

N

N

N

O

O

86

85

Fig. 49. Schematic structure of 85 and 86.

COF FOC

90

AFM, SEM and TEM) of organic, metal–organic and protein–metal organic materials.

Fig. 51. Schematic structure of 90.

6.1. Hg2+ as guest species A lower rim 1,3-di-benzimidazole conjugate of calix[4]arene (92) was synthesized, characterized and is selective towards Hg2+ in aqueous acetonitrile by forming a 1:1 complex. The Hg2+ complex could be well differentiated from its precursor 92 by microscopy.

OH OH

O

O

O

OH OH

For the TEM measurements, a sample of 92 was prepared from ethanol and the study showed the formation of reasonably uniform and well separated spherical clusters (5–10 nm). In the presence of Hg2+ , these further aggregate to form clusters of 40–70 nm size, which, in turn, are connected together to form dumb-bell shaped

O

O

OH OH

O HN

NH HN

O

O

NH N

N

87

HN

NH

HN

NH

O O

88

N

N

89

Fig. 50. Schematic structures of the calixarene conjugates (87, 88 and 89) used for magnetic nanoparticle preparation.

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Fig. 52. (a) Schematic structure of thioester-functionalized calix[8]arene (91). (b) TEM images (from Ref. [93], reproduced with permission from Elsevier Publishers) of (1) Pd, (2) Pt and (3) Ru nanoparticles stabilized by 91.

chains with some branching (Fig. 53) [94] and is reminiscent of cell fusion and transfer of contents between the cells during the division. A structurally characterized lower rim 1,3-di{4antipyrine}amide conjugate of calix[4]arene (93) recognizes Hg2+ in solution among all the other metal ions studied. The nanostructural features, such as, shape and size of the particles obtained using AFM and TEM distinguishes 93 from its Hg2+ complex. All these features are different from those of the simple mercuric perchlorate (Fig. 54) [95]. 6.2. Cu2+ as guest species A structurally characterized, selective fluorescence switch-on sensor, 94 was synthesized and it selectively recognizes Cu2+ as studied in solution. While 94 shows rod like structure of length varying from 4 to 20 ␮m, its Cu2+ complex shows smaller particles (1–2 ␮m) of irregular shape with a smooth surface though these are approximately closer to spherical ones. AFM of 94 shows spherical particles of three different sizes owing to the aggregation. However, this aggregation is severe in case of Cu2+ complex of 94 leading to the formation of large size clusters with size > 250 nm and height > 35 nm (Fig. 55) [96].

Fig. 53. (a) Schematic structure of 92. TEM of (b) 92 and (c) [Hg2+ + 92]. From Ref. [94], reproduced with permission from American Chemical Society.

Fig. 54. (a) Schematic structure of 93. AFM micrographs: (b) and (e) 93; (c) and (f) [93 + Hg2+ ] complex; (d) and (g) Hg(ClO4 )2 . From Ref. [95], reproduced with permission from American Chemical Society.

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Fig. 55. (a) Schematic structure of 94. AFM images of (b) 94 and (c) [Cu2+ + 94]. From Ref. [96], reproduced with permission from Elsevier Publishers.

Fig. 56. TEM images of (a) 94, (b) [Ag+ + 94] and (c) {[Ag+ + 94] + Cys} and their particle size distribution plots as insets. From Ref. [97], reproduced with permission from American Chemical Society.

6.3. Ag+ as guest species The receptor 94 also exhibits high selectivity toward Ag+ by forming a 1:1 complex, among the nine other biologically important metal ions, as studied by fluorescence, absorption, and 1 H NMR spectroscopy. The micrographs in TEM show spherical particles of two sizes (90–150 nm and 180–270 nm) in case of 94. When AgClO4 is added to 94, the size of the particles is reduced drastically to 15–35 nm and the shape is changed to non-spherical, indicating the complexation of 94 with Ag+ where the complex possessing silver ions exhibit dark spots. Upon addition of Cys to the complex, the micrographs are filled with particles of all the three types, viz. 94, [Ag+ + 94], AgClO4 and [Cys + Ag+ ] complex (Fig. 56) [97] as expected.

particles of size ∼50–100 nm which are joined together to form chains (Fig. 58) [99,100]. 6.5. Zn2+ followed by amino acids and proteins as guest species Conjugate 97 is selective towards Zn2+ . [Zn2+ + 97] recognizes Asp, Cys, His and Glu from among the naturally occurring amino acids as demonstrated based on fluorescence, absorption and lifetime measurements. The amino acids present in the proteins also interact with [Zn2+ + 97] resulting in the removal of Zn2+ from this complex [Zn2+ + 97] to result in the formation of aggregates of the protein as demonstrated by absorption, fluorescence spectroscopy and CD studies. In AFM, [Zn2+ + 97] complex exhibits poly-dispersed particles of 70–190 nm size and 4–9 nm height. Clusters of such particles have also been found. When BSA is treated with [Zn2+ + 97],

6.4. Zn2+ as guest species Carboxamidoquinoline appended calix[4]arene-1,3-diconjugate (95) recognizes Zn2+ in solution. The spherical nano-structural features observed for 95 in TEM are being transformed into the Koosh nano-flower like structure when complexed with Zn2+ (Fig. 57) [98]. A structurally characterized lower rim 1,3-di-conjugate of calix[4]arene (96) exhibits selective recognition towards Zn2+ (switch-on) and Ni2+ (switch-off) by showing changes in the fluorescence intensity via forming a 1:1 complex in each case, where both these ions bind simultaneously to 96 in a cooperative manner. The microscopic features of 96 and its Zn2+ and Ni2+ complexes, viz. [Zn2+ + 96] and [Ni2+ + 96] are quite different from each other. In TEM, 96 showed nanotubes or nanorods of length ∼1–2 ␮m and width of ∼200 nm. When Zn2+ was added to 96, discrete particles with sizes ∼100–200 nm were formed. However, in the presence of Ni2+ , the rods are interestingly transformed to small spherical

Fig. 57. (a) Schematic structure of 95. TEM images of (b) 95 and (c) [95 + Zn2+ ]. From Ref. [98], reproduced with permission from Royal Society of Chemistry.

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Fig. 58. (a) Schematic structure of 96. TEM images of (b) 96, (c) [Zn2+ + 96] and (d) [Ni2+ + 96] respectively (from Ref. [100]).

Fig. 59. (a) Schematic structure of [Zn + 97]; AFM pictures of (b) HSA, (c) [Zn + 97] + HSA, (d) BSA, (e) [Zn + 97] + BSA, (f) Jacalin, (g) [Zn + 97] + Jacalin, (h) PNA, (i) [Zn + 97] + PNA. From Ref. [101], reproduced with permission from American Chemical Society.

it exhibited uniformly distributed particles (160–280 nm size and 10–20 nm height) which are roughly double in size when compared to BSA suggesting aggregation. Similar results were observed even with HSA. Such studies carried out in case of two different ␤-sheet proteins, viz. PNA and Jacalin, the particles exhibited size of 85 and 115 nm respectively. Upon treating these lectins with [Zn2+ + 97], the particle size increases dramatically for Jacalin (∼350 nm) whereas a nominal increase was observed (∼165 nm) in case of PNA, both resulted owing to the aggregation (Fig. 59) [101].

6.6. Carboxylate side chains in Asp, Glu and proteins as guest species Lower rim 1,3-di-phenylalanine-peptido-conjugate of calix[4]arene (98) possessing terminal COOH moieties exhibit selective recognition towards Asp, Glu and glutathione (GSH). The Asp complex of 98 forms spherical clusters of 200–300 nm size. Such studies have also been extended to different proteins. In order to confirm the aggregational behavior, further studies were carried out with Ag nanoparticles (AgNP) capped with GSH, since

Fig. 60. (a) Schematic structure of 98. SEM images of (b) 98 and (c) {98 + Asp}; (d) TEM and (e) AFM image of nanospheres formed by {(AgNP–GSH) + 98}; AFM images of (f) BSA, (g) [98 + BSA], (h) HSA, (i) [98 + HSA]. From Ref. [102], reproduced with permission from American Chemical Society.

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98 recognizes COOH moieties of this tripeptide. When 98 was added to AgNP-GSH, the nano-species were further aggregated into spherical clusters (nano-spheres) with diameters ranging from 60 to 120 nm. Similar results have been obtained in AFM studies carried out between {AgNP–GSH} and 98 (Fig. 60) [102].

together with the hydrophobic and electrostatic supramolecular interactions significantly affect the outcome of DNA condensation and cell transfection (Fig. 61) [103]. Similar studies carried out with guanidinium groups attached at the lower rim of calix[4]arenes (108–110) exhibit less toxicity towards cells and significantly higher cell transfection ability as compared to its upper rim analog (111). The role of macrocyclic ring of calixarene and the effect of upper rim substitution on the calixarene unit on DNA binding and condensation properties, cell transfection ability, and toxicity have been monitored (Fig. 62) [104]. Thio-urea attached carbohydrate derivatives were synthesized using methoxycalix[6,8]arene platform (112 and 113), and their aggregational properties in water were studied by 1 H NMR, AFM

7. Bio-molecular interactions with the conjugates of calixarenes by microscopy The upper rim functionalized calix[n]arenes with the guanidinium group (99–107) have been used for binding to DNA and for the cell transfection wherein the studies are being carried out by spectroscopy as well as AFM. Even small variations in size, lipophilicity and conformations of these water soluble compounds

ClNH2+ H2N

H2N

+

Cl+H

2N

NH2

NH2

Cl-

Cl-

NH2

NH2+

H2N NH

HN

HN

NH

NH

Cl-

H2N

NH2 +

ClNH2+

OMe Cl-

HN

OMe

+H N 2

MeO

NH

OMe NH2

O

O

O

O

R

R

R

R

R = C3H7: 4G4Pr-cone R = C6H13: 4G4Hex-cone R = C8H17: 4G4Oct-cone

HN

NH2

99 100 101

NH2

n-3

+

Cl-

n = 4: 4G4Me-mobile n = 6: 6G6Me-mobile n = 8: 8G8Me-mobile NH2 Cl+H N 2

H2N

NH

HN

O

NH2

Cl+H

1

2N

R

HN

NH

O O

Cl+ H2 N

H2N

ClNH2 +

102 103 104

O

NH

HN

ClNH2+

O

O R

O

O

R

R

R

NH2

NH2

4G4Pr-alt

105

2G4Oct-cone: R1 = H 3G4Oct-cone: R1 = NH2 HN

Fig. 61. Schematic structures of the upper rim guanidinium calixarenes.

ClNH 2+

106 107

Cl-

NH2+

A. Acharya et al. / Coordination Chemistry Reviews 256 (2012) 2096–2125

R

O

R

O

O

NH

NH H2N

R

R

+

Cl-

R

R

O

O

O

+

NH

+

HN + H2 N H2N NH2 ClNH2 Cl + Cl-

H2N NH

2123

NH

H2N Cl-

H2N NH2

+

NH2

HN NH2

Cl-

111

R = t-Bu R=H R = Hex

108 109 110

Fig. 62. Schematic structures of the lower rim guanidinium calix[4]arenes.

OH OH

H HO

H HO

OH HO

H

H

H O

OH

HN H H

H O

S

OH

HO

OMe

H H

N H

H

OMe OMe MeO

OH HO

OH

OH OH

H

NH

S

H O

O H

H N

S H N

HN

H HO

HO H

NH

MeO OMe

S

HN

N H S

NH O H

HO HN

S

n-5

H H

NH HO H

H HO HO

OH HO

OH H

O H OH H

n=6 n=8

112 113

Fig. 63. Schematic structures of the upper rim thio-urea linked glyco-conjugates of calix[6,8]arenes.

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and DLS techniques. These macrocyclic structures form aggregates of ∼2–10 nm in the absence of any buffer whereas in the presence of phosphate buffer these form aggregates of ∼200–300 nm and not in the presence of any other anion, such as, Cl− , NO3 − and SO4 2− . The interaction of these glyco-clusters with plasmid DNA was studied, but no DNA condensation was observed. The agglutination occurs only in the presence of concanavalin A (ConA) (Fig. 63) [105]. Similarly calixarene derivatives were used as biolinker molecules to immobilize proteins. The sensitivity and specificity of the used calix-crown chip is much higher when compared to the five other protein attachment agents. Studies showed that by using calix-crown derivatives as molecular linker, better directional immobilization of proteins can be achieved on the solid surface. The calix-crown derivatives and the protein form a host–guest complex. The complexation is driven by electrostatic interaction between calix-crown derivative and the positive amine groups present in the protein and leads to a more efficient site-specific attachment of proteins onto the solid surface [106].

8. Conclusions and future directions It is evident from the literature that the calixarenes have entered into the world of nanoscience and nanotechnology at a fast rate owing to their diverse roles and utilities. A combination of the wide spread synthetic methodologies spurred in the literature with its unique feature of the self-assembly, coupled with their high resolution analytical support, the derivatives of calixarenes find roles in varied materials applications. In this regard, at least some of the calixarene conjugates have already made their mark. Because of the well established derivatization procedures, inherent mobility and flexible cavity size, calixarenes can be modified according to requirement. Thus the conjugates of calixarenes can be tuned to be lipophilic or hydrophilic depending upon the application. Ease of formation of micellar structures is an added advantage. The recent literature is rich with the ion and molecular recognition aspects, wherein the utility of the microscopy methods is indispensable particularly in the characterization of the nano-structural features of the conjugates of the calixarenes and their species of recognition. Such recognition properties can perhaps be enhanced if the conjugates of calixarenes were attached to the solid surfaces and thus extend their use in sensor applications. Calixarene based dendrimers can be tailored to nanometric dimensions and hence can be used in different nanotechnological applications. Calixarene based tube systems can be used as ionophores as well as ditopic systems. Peptido- or glycocalixarenes are yet to be used to their full potential. These are an important class of molecules in the fields of glycomics and glycobiology and thus enter into the field of bionanotechnology. Further, these have been widely used as the anchoring agents on the surfaces of different substrates based on the principles of nanofabrication. Similarly, amphiphilic calixarenes generate complex self-assembly behavior in different solvent media and generate variable nanostructures. Further, the QDs coated with glyco-calixarenes can be used for the selective recognition of different biomolecules, viz. cells, DNA, lectins, etc. In all these, the calixarene based nanoparticles bind selectively to some guest species and finally can deliver the species to the target site based on the pH variations of the local environment. The nano-biosensors are expected to give better selectivity and sensitivity when compared to their conventional molecular counterparts. Currently, the magnetic nanoparticles (MNPs) of the conjugates of calixarenes receive great attention because of their potential applications in nano-medicine and even for their suitability as MRI contrast agents. The magnetic NPs when coupled with suitable conjugates of calixarenes can be employed in cancer treatment (magnetic

hyperthermia). Thus the potential applications that the conjugates of calixarenes offer to the field of nanotechnology are enormous. Recent advances of calixarene derivatives in the field of nanotechnology are mostly focused on the fabrication of calixarene conjugates on the surfaces of solid substrates, such as graphite, gold, silver, Fe2 O3 and TiO2 through the formation of thin films and monolayer arrays. Such fabrication was used to induce sensitivity and selectivity into the organic thin-film transistors, for electronbeam lithography, and for sensing different ionic and molecular species. Calixarene based self-assembled monolayers have been widely used for the guest species of recognition making the path clear for the nanobiosensing. The chiral and phosphonated calixarene derivatives formed nanofibers or nanorods or gels. Thus the future applications of these are directed towards different biomedical applications, such as biocompatible bandages for wounds or as scaffolds for controlled and sustained molecular drug release. Calixarene conjugates have also been used for nanoparticle aggregation studies by host–guest complexation. Different calixarene nanoemulsions have been used for ion extraction studies. Calixarene based vesicles can be used as drug delivery agents owing to their low toxicity and immune response. This enables new applications of these supramolecules in biomedical and pharmaceutical sciences. Thus the potential applications of calixarenes in the field of medical diagnosis are just a step away and soon this challenge is expected to be taken up by the researchers which will open up new methodologies in nano-medicine and nano-therapy. These can also act as nano-transporters towards different biologically important species. In this review, we have tried to look at the current scenario of calixarene derivatives in the perspective of nanoscience and hence we expect that the readers will get a comprehensive view of the subject. Thus, the calixarenes when explored to their best potential, will yield satisfactory results in the field of nanoscience and nanotechnology. Acknowledgments CPR acknowledges the financial support from DST, CSIR and BRNS and Chair Professorship of IIT Bombay. AA and KS acknowledge CSIR and UGC respectively for their fellowships. References [1] C.P. Poole Jr., F.J. Owens, Introduction to Nanotechnology, John Wiley & Sons, NJ, 2003. [2] M.C. Daniel, D. Astruc, Chem. Rev. 104 (2004) 293. [3] C.J. Murphy, A.M. Gole, J.W. Stone, P.N. Sisco, A.M. Alkilany, E.C. Goldsmith, S.C. Baxter, Acc. Chem. Res. 41 (2008) 1721. [4] S.G. Ruehm, C. Corot, P. Vogt, S. Kolb, J.F. Debatin, Circulation 103 (2001) 415. [5] A. Saleh, M. Schroeter, C. Jinkmanns, H.P. Hartung, U. Modder, S. Jander, Brain 127 (2004) 1670. [6] W.C. Chan, D.J. Maxwell, X. Gao, R.E. Bailey, M. Han, S. Nie, Curr. Opin. Biotechnol. 13 (2002) 40. [7] D.R. Larson, W.R. Zipfel, R.M. Williams, S.W. Clark, M.P. Bruchez, F.W. Wise, W.W. Webb, Science 300 (2003) 1434. [8] C.D. Gutsche, Calixarenes, Royal Society of Chemistry, Cambridge, UK, 1989. [9] L. Mandolini, R. Ungaro, Calixarenes in Action, Imperial College Press, London, 2000. [10] R. Joseph, C.P. Rao, Chem. Rev. 111 (2011) 4658. [11] H.J. Kim, M.H. Lee, L. Mutihac, J. Vicens, J.S. Kim, Chem. Soc. Rev. 41 (2012) 1173. [12] A.W. Coleman, S. Jebors, P. Shahgaldian, G.S. Ananchenkoc, J.A. Ripmeesterc, Chem. Commun. (2008) 2291. [13] P. Shahgaldian, M. Cesario, P. Goreloff, A.W. Coleman, Chem. Commun. (2002) 326. [14] M. Pojarova, G.S. Ananchenko, K.A. Udachin, M. Daroszewska, F. Perret, A.W. Coleman, J.A. Ripmeester, Chem. Mater. 18 (2006) 5817. [15] D.N. Polovyanenko, E.G. Bagryanskaya, A. Schnegg, K. Mobius, A.W. Coleman, G.S. Ananchenko, K.A. Udachine, J.A. Ripmeester, Phys. Chem. Chem. Phys. 10 (2008) 5299. [16] G.S. Ananchenko, K.A. Udachin, A. Dubes, J.A. Ripmeester, T. Perrier, A.W. Coleman, Angew. Chem. Int. Ed. 45 (2006) 1585.

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