This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Article Cite This: ACS Omega 2018, 3, 16989−16999
http://pubs.acs.org/journal/acsodf
Physicochemical and Ion-Sensing Properties of BenzofurazanAppended Calix[4]arene in Solution and on Gold Nanoparticles: Spectroscopy, Microscopy, and DFT Computations in Support of the Species of Recognition Bhawna Uttam, M. Althaf Hussain, Sunita Joshi, and Chebrolu Pulla Rao*
Downloaded via UNIV OF RHODE ISLAND on December 12, 2018 at 10:28:34 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Bioinorganic Laboratory, Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India S Supporting Information *
ABSTRACT: A calix[4]arene conjugate (L) functionalized at the lower rim with a benzofurazan fluorophore (NBD) and at the upper rim with a thioether moiety has been synthesized and characterized by 1H NMR, 13C NMR, and mass spectrometry techniques. Both the absorption and emission spectral data for L in different solvents exhibited progressive changes with an increase in polarity. Ion recognition studies were performed by absorption and fluorescence spectroscopy using 10 different metal ions. Among these, Hg2+ exhibited greater changes in these spectra, whereas Cu2+ showed only significant changes and all other ions showed no change in the spectral features. Although the Hg2+ has dominant influence on the spectral features and provides a detection limit of 56.0 ± 0.6 ppb, the selectivity was hampered because of the presence of the derivatizations present on both the rims of L for ion interaction in solution. Therefore, L was immobilized onto gold nanoparticles (AuNPL’s) so that the upper rim derivatizations anchor onto the gold surface through Au−S interactions, and this leaves out only the lower rim NBD derivatization for interaction with ions selectively. The AuNPL’s were characterized by transmission electron microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (XPS) analyses. The surface characteristics were analyzed by contact angle measurements. The AuNPL’s exhibit greater selectivity and enhanced sensitivity for Hg2+ ions with a lowest detection limit of 48.0 ± 0.8 ppb. The immobilization of L onto AuNPs was reflected in the corresponding fluorescence lifetime values, and the addition of Hg2+ to either L or AuNPL showed fluorescence quenching. The reversible recognition of Hg2+ by L was demonstrated by titrating L or AuNPL with Hg2+ followed by tetra-butyl ammonium iodide for several cycles. The structural features of Hg2+-bound species were demonstrated by density functional theory computations and were supported by the XPS data. The Hg2+ induces aggregated fibrillar morphology into supramolecular L, as demonstrated by microscopy when Hg2+ was added either to L or to AuNPL, supporting aggregation-caused quenching.
■
colorimetric and fluorometric receptors for ion detection.9−13 A number of receptors are reported in the literature for Hg2+ ions;14−26 however, those based on the calixarene platform are scant.27−32 In the literature, benzofurazan (NBD) has been explored for its fluorophoric property,33−37 though it is possible to use its N- and O-ligating centers for binding transition metal ions. Therefore, it is of interest to observe the selective ion binding properties of the fluorophoric NBD moiety when appended onto the lower-rim arms of calix[4]arene owing to the supramolecular nature of the resultant conjugates. To our knowledge, there are only a few reports in the literature in which NBD is attached to the calix[4]arene platform. However, the calixarene conjugates with bifunctional
INTRODUCTION Environmental contaminants, such as heavy metal ions, have adverse effects on living organisms.1 Among these, the mercury contamination is potentially hazardous and it can be easily converted into organomercury species such as methyl-mercury via biotic and abiotic pathways and accumulates in living organisms.2−5 Exposure to methyl-mercury leads to cardiovascular and neurological disorders, damages the DNA, and impairs mitosis.6−8 Therefore, simple, rapid, and efficient chemosensors are needed to detect low levels of mercury contamination in toxicological, biological, and environmental samples. For this, calixarenes are important owing to their welldefined structure and conformation and amphiphilic nature because they possess an upper-rim hydrophobic core and lower-rim hydroxyl groups. Furthermore, calix[4]arene is an ideal platform that can be easily modified by attaching a suitable fluorescent binding core for the development of © 2018 American Chemical Society
Received: October 17, 2018 Accepted: November 28, 2018 Published: December 11, 2018 16989
DOI: 10.1021/acsomega.8b02848 ACS Omega 2018, 3, 16989−16999
ACS Omega
Article
Scheme 1. Synthesis of L; C4A is p-tert-Butylcalix[4]arenea
mode and with the NBD moiety have not been explored in the literature and hence the present study is of great relevance. Recently, the lower rim of calix[4]arene was modified by the NBD moiety, where the conjugate was used for the detection of either a cation, such as Ag+, or an anion, such as F− ions.38,39 In another report, four folic acid units are appended at the upper rim and an NBD moiety at the lower rim of calix[4]arene, where the conjugate was demonstrated to be internalized by the cancer cells.40 Because of the requirement of special features for selective sensing of mercury, a bifunctional calix[4]arene conjugate (L) was designed (Figure 1) to have a binding core within a fluorophore at the lower rim
a
(i) 3-Bromopropyl pthalimide, K2CO3, acetonitrile, 24 h at reflux; (ii) HMTA, TFA, reflux for 36 h; (iii) NaClO2, NH2SO3H, CHCl3/ acetone (1:1 volume ratio), rt for 24 h; (iv) EDCI·HCl, CH2Cl2, Et3N, rt for 24 h; (v) hydrazine hydrate, C2H5OH, 12 h at reflux; (vi) 7-chloro 4-nitro benzofurazan, pyridine, C2H5OH, rt for 24 h. R = t Bu.
Figure 1. Schematic representation of the design of the receptor, L, and its immobilization onto AuNPs.
while having −SMe terminals at the upper rim. The upper rim −SMe terminals will help anchor the molecule onto the noble metal surface such as gold to bring better ion selectivity as we have already demonstrated that the anchoring onto metal surface would bring sensitivity and/or selectivity in some cases.41−44 L has been used as a sensor for Hg2+ and Cu2+ ions in solution and only for the Hg2+ ion when L is coated onto the surface of the gold nanoparticles (AuNPL’s). While the receptor molecule tumbles freely in solution, the same is immobilized when coated onto AuNPs and thereby provides an access only to the lower-rim binding core. This is expected to bring a difference in the sensitivity and selectivity of the receptor toward the guest species on going from solution to that anchored onto the AuNP surface.
λem are red-shifted by 41 and 57 nm, respectively, suggesting a positive solvatochromism for L. The emission intensity of L is enhanced by 16-, 31-, and 40-fold in ethanol, acetonitrile, and tetrahydrofuran (THF), respectively, as compared to its intensity in water (Figure S02). Thus, the solvent-dependent fluorescence intensity of L follows a trend, that is, water ≪ methanol < n-hexane < ethanol < dichloromethane ~ DMSO ~ carbontetrachloride < toluene < DMF ~ acetonitrile < THF. Because the solvatochromic behavior of L in ethanol is midway between the two extremes, ethanol has been used as the solvent for further studies. Metal Ion Recognition by L in Ethanol Using Spectroscopy. All of the solution studies of ion recognition were carried out in ethanol. To evaluate the binding ability of L toward ions, such as Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Hg2+, fluorescence spectral titrations were carried out (Figure S03). During the titration of L with Hg2+, the fluorescence intensity is quenched, and the extent of quenching is dependent on the concentration of Hg2+ added, as can be noticed from Figure 3a,b. Initially, the color of the receptor L in ethanol is yellow, and it turns to pinkish-orange upon Hg2+ addition. Because the Job plot exhibited a broad peak portion, the complex formed between Hg2+ and L can be fitted to either 1:1 or 1:2 (Figure S04a) and the minimum detection limit for Hg2+ is 56.0 ± 0.6 ppb (141 nM) (Figure S12a). All of the other ions showed no significant change in the fluorescence intensity except Cu2+, where the intensity is enhanced by ∼15−20% as compared to that of the receptor L alone (Figure S04b). The selectivity of Hg2+ has been addressed by carrying out the titration in the presence of other metal ions as competitors, and no significant change in the fluorescence intensity was observed, supporting that none of the other ions compete for L when it is bound to Hg2+ (Figure S05a). This suggests that L can be used as a selective sensor for Hg2+ in the presence of these metal ions studied.
■
RESULTS AND DISCUSSION The receptor L is synthesized by going through several consecutive steps starting from p-tert-butylcalix[4]arene (C4A), as given in Scheme 1. All of the precursors and the receptor molecule were characterized thoroughly (SI01). The presence of a pair of doublets observed at 3.45 and 4.34 ppm in 1 H NMR for the bridge-CH2 of calix[4]arene in CDCl3 confirms that calix[4]arene exists in cone conformation. Physicochemical Studies of L Based on Spectroscopy. Because the absorption and emission characteristics of NBD are known to be dependent on solvent polarity,45 the spectra of L were measured in different solvents, such as n-hexane, carbontetrachloride, toluene, tetrahydrofuran, dichloromethane, N,N-dimethylformamide, dimethylsulfoxide, acetonitrile, ethanol, methanol, and water, and the corresponding spectra are given in Figure 2a,c. The λabs or λem vs relative polarity plot exhibits generally an increasing trend with increasing polarity of the solvents. We have made use of the concept of relative polarity that has been reported in the literature.46,47 A comparison of the data for the two extremes of polarity, viz., CCl4 and water, supports that both the λabs and 16990
DOI: 10.1021/acsomega.8b02848 ACS Omega 2018, 3, 16989−16999
ACS Omega
Article
Figure 2. (a, c) Absorption and fluorescence spectral traces of L, respectively, in different solvents. (b, d) Plot of λabs or λem vs relative polarity of the solvent.
studied by SEM in both the presence and the absence of Hg2+. Because ethanol and THF have a larger polarity difference, these were chosen as solvents for the preparation of the samples used for microscopy studies. In ethanol, it was observed that L exhibits well-separated, rodlike structures of size 50−60 nm and that upon addition of Hg2+ aggregation is induced in L, resulting in triangular shuttle-cock-like structures (Figure S07a,b). In the case of THF, these rods join together to form fibrillar structures (Figure 5a) and upon addition of Hg2+ dense and twisted fibrils were obtained, supporting ioninduced supramolecular aggregation in L (Figure 5d). The fibril formation by L followed by aggregation in the presence of Hg2+ ions was further proved by other microscopy techniques, such as TEM and AFM. Similar to SEM, fibrillar structures of L in THF were also noticed even in TEM and AFM (Figure 5b,c). In the presence of Hg2+, the aggregation of the fibrils increases their density, and this is very well supported by SEM (Figure 5e,f). Thus, the fluorescence quenching observed in L upon addition of Hg2+ is attributable to the metal-ion-induced aggregation-caused quenching (ACQ) as shown in Figure 5g. L-Capped Gold Nanoparticles (AuNPL). The receptor L was designed in such a way that the two −SMe groups present at the upper rim of calixarene can easily bind to the gold nanoparticles through Au−S interactions. To study the ion selectivity of L on the surface, receptor L was coated using citrate-capped gold nanoparticles (AuNPcit’s) by the ligand exchange method, as given in the Experimental Section. Both the AuNPcit and AuNPL were characterized by absorption spectra, TEM, XPS, and contact angle measurements, as given in Figure 6a,b. The AuNPcit shows an SPR band at 530 nm, indicating the formation of the nanoparticles. The formation of AuNPL was confirmed by the presence of both the SPR band at 530 nm and that of L at 465 nm, supporting that the nanoparticles possess L. The EDAX data given in Figure S08 supports the coating of L onto the gold nanoparticles. X-ray photoelectron spectroscopy (XPS) measurements on AuNPL revealed atom % of 8.5, 62.0, 20.0, 6.5, and 3.0, respectively, for Au, C, O, N, and S, suggesting the grafting of the conjugate of calix[4]arene (L) onto AuNPs. The binding energies of Au 4f peaks observed at 84.7 (4f7/2) and 88.1 eV (4f5/2) (Figure 6c)
Figure 3. (a) Fluorescence spectral traces for the titration of L with Hg2+ in ethanol during the addition of 0 to 10 equiv of Hg2+. The inset shows the color of the solutions of L and {L + Hg2+}. (b) Plot of [M2+]/L vs fluorescence intensity at λ532 (nm).
The absorption spectrum of L in ethanol exhibits four major bands at 210, 264, 335, and 465 nm, where the first two arise from the calix[4]arene platform and the latter two arise from the NBD moiety (Figure S05b). Among all of the ions studied, Hg2+ showed maximum changes in the absorption spectrum along with the formation of a new band at 530 nm at the cost of the absorbance of the band at 465 nm. During this process, the color of the solution changes from yellow to pinkish-red, supporting the formation of the complex of L by Hg2+ (Figure 4a,b). All of the other ions showed no significant change in the absorption spectra of L (Figure S06), except Cu2+ that showed marginal changes in the bands at 332 and 465 nm (Figure 4c). The (A−A0) plot for all of the three bands suggests that the Hg2+ selectively interacts with L while generating a chargetransfer band at ∼530 nm. However, no such charge-transfer band is observed in the case of Cu2+, supporting the fact that L can uniquely detect Hg2+ over all other ions studied on the basis of absorption spectroscopy and thus L is selective for Hg2+ (Figure 4d). The effect of a counter anion on L has been studied by employing chloride, nitrate, acetate, and perchlorate salts of Hg2+, and it was found that the fluorescence quenching was half in the case of HgCl2 and Hg(NO3)2 as compared to that in the acetate and perchlorate salts (Figure S05c). The sensitivity of L for the Hg(ClO4)2 salt does not alter even in the presence of other salts possessing other anions, such as chloride, nitrate, and acetate (Figure S05d). Hg2+-Induced Aggregation in L Studied by Microscopy. As the conjugates of calixarenes are known to exhibit supramolecular features,48,49 the aggregation behavior of L was
Figure 4. (a) and (c) Absorption spectral traces obtained during the titration of L with Hg2+ and Cu2+, respectively, upon addition of 0−10 equiv of the corresponding ion in ethanol. (b) Plot of absorbance at λ530 vs {[L]/Hg2+}. (d) Bar diagram showing the differential absorbance (A−A0) for each metal ion at 332 nm (black), 465 nm (red), and 530 nm (blue). 16991
DOI: 10.1021/acsomega.8b02848 ACS Omega 2018, 3, 16989−16999
ACS Omega
Article
Figure 5. Microscopy images of the samples taken in THF but measured from their thin films on Al foil, copper grid, mica sheet, respectively, for SEM (a, d; the bar is 100 nm), TEM (b, e; the bar is 1 μm) and AFM (c, f; total 10 μm). (a), (b) and (c) for L, and (d), (e) and (f) for {L + Hg2+}. (g) Schematic representation of the Hg2+-induced aggregation in fibrils of L.
Figure 6. (a) Absorption spectra of L (black), AuNPcit (red) and AuNPL (blue) in ethanol. (b) Images showing the wettability of the surface by contact angle measurements: 1. AuNPcit; 2. AuNPL; 3. L. (c) XPS spectra of AuNPL for Au_4f.
are in agreement with the data reported for alkanethiols assembled on gold nanoparticles.50 The contact angle of the water droplet on AuNPcit is 15°, whereas it is ∼90° on the receptor L. When L is immobilized onto AuNPs to give AuNPL, the contact angle changes to ∼68°, supporting the coating of L on AuNPs. Ion Recognition by AuNPL. Absorption spectral titrations were carried out further to check the sensitivity and selectivity of L-coated gold nanoparticles, viz., AuNPL, for divalent transition metal ions, viz., Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Hg2+ (Figure S09). In solution, the receptor was sensitive and selective for Hg2+ ions; however, it weakly interacted with Cu2+. When the same L was coated on nanoparticles, viz., AuNPL, the sensitivity for the Hg2+ ions was greater even when a lower concentration of Hg2+ was added. Both the 464 and 530 nm bands showed a decrease in absorbance initially, and the band at 530 nm completely diminished with an increase in the Hg2+ concentration, and this is associated with the formation of a new band at 615 nm, suggesting the aggregation of AuNPL’s (Figure 7a,b). Whereas the Cu2+ titration with simple L shows considerable changes in the absorption spectra, in the case of AuNPL, the same ion shows no significant change in the absorbance; thus, the study supports enhanced selectivity for Hg2+ of L upon anchoring onto the AuNP surface. In all of the other metal ions studied, no absorption band was observed at 615 nm, implying that no other M2+ forms a complex with AuNPL (Figure 7e). Fluorescence spectral titrations were also carried out with all of these metal ions (Figure S10), and it was observed that the intensity was quenched when AuNPL was titrated with Hg2+. The Hg2+ concentration required for complete quenching is lower in the case of AuNPL as compared to that for simple L in solution, supporting that the sensitivity of L is enhanced when it is anchored onto the Au surface (Figure 7c,d,f). The detection limits obtained for Hg2+ on the basis of the
Figure 7. (a) Spectral traces obtained during the titration of AuNPL with Hg2+ by absorption spectroscopy. (b) Plot of the absorbance vs [Hg2+]/[AuNPL] for the bands 465 (black), 520 (red), and 615 (blue) nm. (c) Spectral traces obtained during the titration of AuNPL with Hg2+ ions by fluorescence spectroscopy. (d) Plot of fluorescence intensity vs [Hg2+]/[AuNPL]. (e) Plot of absorbance at 615 nm as a function of metal ions. (f) Plot of fluorescence intensity as a function of metal ions.
fluorescence intensity data by simple L and AuNPL are 56.0 ± 0.6 ppb (141 nM) and 48.0 ± 0.8 ppb (121 nM), respectively (Figure S11a,b). The data was derived on the basis of 3σ/m, where σ corresponds to the standard deviation of the blank measurements and m is the slope of the plot of the intensity vs concentration of the sample. Thus, the detection limit for L is largely greater by 2σ when compared to that observed for AuNPL, supporting that the detection of Hg2+ by AuNPL is certainly more sensitive than that by L. This indicates that when L is supported on a solid surface, its ion sensitivity increases. Thus, the sensitivity of Hg2+ detection is greater by 115% in the case of AuNPL as compared to that for the simple L. The effect of counter anion on AuNPL has been studied by employing chloride, nitrate, acetate, and perchlorate salts of Hg2+, and it is found that the fluorescence intensity does not change to any significant extent, as can be noted from the spectral traces (SI12, Figure S12). The TEM micrographs of AuNPcit (Figure 8a) show spherical particles of size 17.1 ± 3.8 nm. The HRTEM 16992
DOI: 10.1021/acsomega.8b02848 ACS Omega 2018, 3, 16989−16999
ACS Omega
Article
Figure 9. (a) and (c) Fluorescence spectral traces obtained during the titration of [L + Hg2+] and [AuNPL+Hg2+], respectively, with tetrabutyl ammonium iodide (TBAI). (b) Fluorescence intensity obtained during the titration of L with Hg2+, followed by addition of TBAI for four consecutive cycles: (i) L, (ii) {(i) + Hg2+}, (iii) {(ii) + TBAI}, (iv) {(iii) + Hg2+}, (v) {(iv) + TBAI}, (vi) {(v) + Hg2+}, (vii) {(vi) + TBAI}, (viii) {(vii) + Hg2+}, and (ix) {(viii) + TBAI}. (d) Fluorescence intensity data for a recycling experiment similar to that given under (b) but starting with AuNPL. Figure 8. TEM micrographs of AuNPcit with scale bars (a) 50 and (d) 2 nm, of AuNPL with scale bars (b) 50 and (e) 2 nm, and of AuNPL+Hg2+ with scale bars (c) 0.2 μm and (f) 50 nm. (g−i) Diffraction patterns of AuNPcit, AuNPL, and AuNPL+Hg2+ respectively. (j), (k), and (l) SEM micrographs of AuNPcit (100 nm), AuNPL (1 μm), and AuNPL+Hg2+ (100 nm), respectively.
quenching (OFF switch) and upon addition of TBAI the fluorescence intensity is regained and hence acts as fluorescence ON switch. The ON/OFF behavior of the receptor is clearly seen by the changes observed in the fluorescence intensity, and this is given in Figure 9b. The reversible nature can also be noticed from the visual color change (SI14, Figure S14). Similar titration studies were also carried out to show the reversibility of L-coated nanoparticles (AuNPL) for sensing of Hg2+, and it was found that AuNPL also shows reversible behavior. This was shown for four consecutive cycles (Figure 9c,d). Thus, both free L and surface-coated L, i.e., AuNPL, show reversible sensing of Hg2+. Fluorescence Lifetime Measurements of L, AuNPL, and Its Hg2+ Complexes. Because the fluorescence excitedstate lifetimes are sensitive to the structure and dynamics of a fluorophore, the decay profiles of receptor L and those after anchoring it onto the gold nanoparticles (AuNPL) were measured, in both the absence and the presence of Hg2+ using the time-correlated single-photon counting (TCSPC) method, as can be noticed from Figure 10. The decay data were found
image and the diffraction pattern were also obtained (Figure 8d,g). Although the size of the AuNPs did not change after the addition of L, fibriltype structures were noticed in which the AuNPs were dispersed onto these fibrils (Figure 8b,e,h). Upon addition of Hg2+ to AuNPL, the discrete particles connected the fibrils and the fibrils were further aggregated, as can be seen from the TEM images given in Figure 8c,f,i. The crystallinity of the AuNPs decreased after coating of L on them, and the crystallinity was lost after the addition of Hg2+ (Figure 8g,h,i). The aggregation on AuNPL after addition of Hg2+ was further supported by SEM (Figure 8j−l) and the DLS data (SI13, Figure S13). The size distribution of AuNPL (340 ± 8 nm) measured by DLS clearly supported strong aggregation when compared to that of the precursor AuNPcit, which showed only 32 ± 1 nm size particles. Upon addition of Hg2+, the AuNPL’s further aggregated by an order of magnitude higher, exhibiting an average size of 2470 ± 20 nm. Thus, the aggregation is increased on going from AuNPcit → AuNPL → AuNPL+Hg2+ and finally exhibits a fiberlike structure upon addition of Hg2+. Reversibility. The reversible behavior of the sensing of Hg2+ by L and AuNPL has been demonstrated by the titration of each of L and AuNPL with Hg2+ followed by tetra-butyl ammonium iodide (TBAI) (Figure 9). As the iodide ion possesses greater affinity for Hg2+, it forms a complex, [HgI4]2−, where the Hg2+ is removed from the [HgL] complex and the resultant free L shows enhanced emission intensity. Further addition of Hg2+ to this results in fluorescence quenching because of the formation of [HgL]. With an increase in the concentration of TBAI, the fluorescence intensity increases (Figure 9a) and the intensity regains the level that it initially showed (when no Hg2+ ions was present), suggesting a complete reversibility of the receptor and its Hg2+ complex. Thus, the reversibility of the receptor L was shown for four cycles and the corresponding data is given in Figure 9b. In this, the initial addition of Hg2+ results in fluorescence
Figure 10. Time-correlated single-photon counting (TCSPC) data for L, L + Hg2+, AuNPL, and AuNPL+Hg2+.
to fit with triexponentials, resulting in three species. The triexponential decay observed in the case of the calix[4]arene conjugate L would result from the conformational flexibility of the arms of this supramolecular species, and the corresponding data including the weighted average (τ) are given in SI15 and Table S01. The average lifetime “τ” of the receptor L in ethanol decreases by 25% from 5.6 to 4.2 ns in the presence of Hg2+ ions. However, when L is coated onto the gold 16993
DOI: 10.1021/acsomega.8b02848 ACS Omega 2018, 3, 16989−16999
ACS Omega
Article
Figure 11. Computational results obtained at the B3-LYP/6-31G(d,p)/SDD(Hg2+) level of calculations: (a) optimized structure of L and (b) optimized structure of the complex, [L − Hg2+]. (c) and (d) Primary coordination sphere around Hg2+ in the complex. The numbers shown in (d) correspond to the metal−ligand distances in the coordination core in Å. The bond angles of the coordination core (in degrees) are as follows: O1HgO2 = 138; O1HgN1 = 55; O1HgN2 = 108; O2HgN1 = 126; O2HgN2 = 56; and N1HgN2 = 70. (e) Standard trigonal bipyramidal geometry with one vacant site in the axial position. The hydrogens attached to the carbon atoms are not shown for clarity (color coding for the atoms: cyan, C; fluorescent green, N; dark green, H; purple, O; gold, S; and red, Hg).
nanoparticles (AuNPL’s), the average lifetime of the coated L decreases to 3.3 ns, supporting that the Au surface quenches the fluorescence. When Hg2+ is added to AuNPL, the lifetime of the fluorophore is further decreased to 2.8 ns and hence exhibits only 15% lowering instead of 25%, which is observed in the case of free L, supporting that L is immobilized on the nanoparticle surface. Binding Nature of Hg2+ in the Complex by Computational Study. The optimized structures for the receptor L and its 1:1 mercury ion complex [L − Hg2+] at the B3-LYP/631G(d,p)/SDD(Hg2+) level are given in Figure 11a,b. As a result of the complexation of L by Hg2+, the 1:1 complex formed is stabilized by 331 kcal/mol and the stabilization is 325 kcal/mol upon applying BSSE correction. In the complex, the Hg2+ ion is stabilized by four noncovalent interactions arising from two nitrogens and two oxygens by exhibiting a coordination core of HgN2O2 (Figure 11c). All of these ligations arise from both the NBD moieties attached as terminal units to both the arms. Whereas the two oxygen ligations are from the two terminal NO2 groups, the two nitrogen ligands are from the benzofurazan moiety. All of the Hg···O (2.742 and 2.835 Å) and Hg···N (3.072 and 3.200 Å) distances support the noncovalent interactions present between Hg2+ and the corresponding ligating centers on L. The angles in the primary coordination sphere range from 55 to 138°, and the geometry at the Hg2+ center is neither tetrahedral nor square planar but fits relatively better to a distorted trigonal bipyramidal geometry where one of the apical positions is vacant (Figure 11d). The structure of the complex thus obtained from the computational study supports the changes observed in the XPS data for 1s of N and O in comparison to those obtained for the receptor (Figure 11). The interaction of the NBD moiety by Hg2+ observed in the complex is expected to alter the absorption and emission spectral characteristics of this moiety. Only in the titration of L by Hg2+, a new band is observed at ∼540 nm because of the charge-transfer transition between NBD and Hg2+, and the fluorescence intensity is quenched as expected because of the heavy atom effect. Both the Mulliken and NBO calculations supported the decrease in the charge at the metal center substantially when Hg2+ interacts with L in the complex (SI16, SI17, Figure S15, Tables S02, S03, and S04).
Because the Job plot is not decisive in determining whether the complex formed between Hg2+ and L is 1:1 or 1:2, twolayer ONIOM computations were performed with the 1:2 complex as per the details given under the Experimental Section. The 1:2 complex thus optimized using two-layer ONIOM computations has been given in SI18 and Figure S16, and the coordination core is given in Figure 12. Whereas the
Figure 12. (a) Hg2+ binding core obtained from two-layer ONIOM computations for optimizing the 1:2 complex, where (i) and (ii) are the NBD groups from each arm of one molecule and (iii) and (iv) are NBD and the upper-rim part of another molecule coordinating to Hg2+. (b) Octahedral coordination around Hg2+ giving the HgN2O4 core. Corresponding metal ion−ligand distances are given in Å in the figure, and their bond angles (in degrees) are as follows: N1−Hg−N2 = 177; N1−Hg−O1 = 110; N1−Hg−O2 = 92; N1−Hg−O3 = 78; N1− Hg−O4 = 72; N2−Hg−O1 = 72; N2−Hg−O2 = 87; N2−Hg−O3 = 103; N2−Hg−O4 = 106; O1−Hg−O2 = 94; O1−Hg−O3 = 73; O1− Hg−O4 = 155; O2−Hg−O3 = 159; O2−Hg−O4 = 111; and O3−Hg− O4 = 84.
1:1 complex is formed between the NBD moieties of both the arms giving rise to the HgN2O2 core, the 1:2 complex is formed between the arms of two molecules of L. Thus, in the 1:2 complex, the HgN2O4 core is formed using three NBD moieties of both the molecules in addition to the involvement of upper rim arm amide “O” binding. Whereas the geometry in 1:1 is distorted trigonal bipyramidal with a vacant site, it is distorted octahedral in the 1:2 complex. In the 1:2 complex, the Hg2+···N/O distances range from 2.162 to 2.688 Å and the angles range from 72 to 110 for the orthogonal bonds and 155 to 177° for the trans bonds with average values of 81 and 164°, respectively, all supporting a highly distorted octahedral 16994
DOI: 10.1021/acsomega.8b02848 ACS Omega 2018, 3, 16989−16999
ACS Omega
Article
Figure 13. XPS spectra of N 1s and O 1s: (a) and (b) for L; (c) and (d) for AuNPL; (e) and (f) for L + Hg2+; and (g) and (h) for AuNPL+Hg2+. The deconvoluted spectra are shown in different colors.
Scheme 2. Schematic Representation of the Physicochemical and Sensing Features of L and AuNPL
HgN2O4 core. Thus, the examination of the Hg2+···O/N bond length data supports that three of these are covalent, whereas all four Hg2+···O/N are noncovalent in the case of the 1:1 complex. The single-point energy calculations carried out on the optimized 1:2 complex yield 25−30% greater stabilization for the HgN2O4 core over its 1:1 counterpart, i.e., the HgN2O2 core, and the increase is commensurate with the number and nature of additional interactions present in the former. Supporting the Binding of Hg2+ in the Complex by XPS Data. To validate the binding features noticed from the computational study, XPS spectra were measured. From the XPS data, it was observed that the binding energy peaks for N_1s and O_1s arise from the benzofurazan moiety and that these were shifted upon Hg2+ addition. The N_1s band for L showed three peaks upon deconvolution, and these peaks were observed at 405.97 (NO2), 403.30 (N−O−N), and 401.24 eV (CONH, NH) (Figure 13a,b). Upon addition of Hg2+, the peaks from NO2 and N−O−N shifted to 404.37 and 403.12 eV, respectively, whereas the area under the N−O−N peak increased. Similar changes were also observed between AuNPL
and AuNPL+Hg2+ (Figure 13c,d) as expected. The O_1s peak could also be fitted to three species in XPS spectra of L, and these were observed at 535.86 (NO2), 533.01 (N−O−N), and at 531.82 eV (CONH) (Figure 13e). Upon addition of Hg2+, the peak from NO2 was shifted from 535.86 to 534.07 eV and that observed at 531.82 eV shifted to 532.14 eV with an increase in its area in the case of L (Figure 13f), suggesting the interaction of Hg2+ with the oxygen of the NO2 group. Similar XPS spectral changes were observed between AuNPL and AuNPL+Hg2+ (Figure 13g,h) as expected.
■
CONCLUSIONS AND COMPARISONS Di-Benzofurazan-appended calix[4]arene (L) having surface binding motifs at alternate positions of calix[4]arene has been synthesized and characterized by 1H NMR, 13C NMR, and mass spectrometry (HRMS). In L, benzofurazan has been used as both the fluorophore and the ligand that binds to the metal ion. The highlights of all of the studies carried out to demonstrate the physicochemical and the metal-ion-sensing 16995
DOI: 10.1021/acsomega.8b02848 ACS Omega 2018, 3, 16989−16999
ACS Omega
■
aspects of both L and AuNP-bound L (i.e., AuNPL) are listed in Scheme 2. L has been shown for its solvent-dependent absorption and emission characteristics. With an increase in the polarity of the solvent, the λabs or λem is red-shifted. The recognition ability of the receptor L in ethanol has been studied with divalent transition metal ions and some toxic ions. Among all of the metal ions, the receptor showed strong interactions with Hg2+ and weak interaction with Cu2+. The interaction of Hg2+ with L resulted in a visible color change from yellow to pinkish-red along with the formation of a new band at 530 nm in absorption spectroscopy, and the fluorescence intensity is quenched upon addition of Hg2+. The interaction of the Hg2+ with L has also been shown by TEM, SEM, and AFM microscopy techniques, which suggest that the fibrillar structures of the receptor L are further aggregated upon addition of Hg2+, suggesting the aggregationcaused quenching (ACQ) of L. Because both the rim groups are exposed to the environment in solution, they lead to a lower selectivity as they exhibit differential interaction with Hg2+ vs Cu2+. Therefore, the receptor was coated onto the gold nanoparticle surface (AuNPs) through Au−S interactions using upper rim −SMe groups. As a result of this, only the lower rim derivatizations are better exposed to the environment, thus restricting the tumbling motion of the receptor L and thereby enhancing its selectivity toward ion sensing. The AuNPL’s were fully characterized by UV−visible spectrophotometry, TEM, and XPS measurements. The elemental mapping done through TEM further supported the presence of L on these AuNPs. The hydrophobicity or hydrophilicity of the surface was probed by contact angle measurements. Among all of the metal ions studied, AuNPL is selective for Hg2+ ions only, and the sensitivity of the receptor increases upon immobilization of L onto the AuNPs. This was also reflected in their minimum detection limits, wherein simple L shows 56.0 ± 0.6 ppb, whereas the AuNP-coated L, i.e., AuNPL, shows 48.0 ± 0.8 ppb; thus, a sensitivity increase of 15% was noticed. A comparison of the literature reports on Hg2+ detection by calixarene conjugates with the present one clearly supports that L shows lower detection limit and hence higher sensitivity toward Hg2+.24,27−30,36 The fluorescence lifetime measurements clearly supported the anchoring of L onto the AuNPs, and it was further reflected in fluorescence quenching when either L or the AuNPL’s were titrated with Hg2+ ions. Upon addition of the Hg2+ to AuNPL, the aggregated species were seen in SEM and TEM, and this was further supported by the DLS data. The complexes of L and AuNPL with Hg2+ are reversible in nature. The reversibility studies have been performed using tetra-butyl ammonium iodide with both the complexes, and as a proof of concept, the reversibility was demonstrated for four cycles. The Hg2+ forms a noncovalent complex with L, wherein such interactions are extended to N- and O-ligation from each of the arm using the NO2 and the NBD groups to result in a HgN2O2 coordination core as a species of sensing, as derived on the basis of DFT studies. As the Job plot is inconclusive for suggesting the formation of either 1:1 or 1:2, the latter was also modeled by computational studies to have the HgN2O4 core. The structure of the complex obtained from the computational studies was further supported by the XPS data analysis. The physicochemical features including solvatochromism, as well as the sensing features, for L and its surface-immobilized counterpart, viz., AuNPL, can be easily understood from Scheme 2.
Article
EXPERIMENTAL SECTION
The perchlorate salts of all of the metal ions were procured from Sigma-Aldrich. All of the solvents used for the spectroscopy studies were of analytical reagent grade and were distilled and dried by standard procedures immediately before use, Milli-Q water is being used. The 1H NMR and 13C NMR (400 MHz Bruker NMR Spectrometer), ESI-MS (in positive ion mode on MaXis Impact-Bruker), UV−vis absorption (Cary 100 Bio UV−Vis spectrophotometer), and fluorescence emission (Varian-Cary eclipse fluorescence spectrophotometer) spectra were measured. Stock solutions of all of the metal salts were prepared in ethanol at 6 × 10−4 M. All of the fluorescence and absorption titrations were carried out in a 1 cm quartz cell using 100 μL of L, and the total volume in each measurement was made up to 3 mL to give a final concentration of L of 20 μM. All of the fluorescence titrations were carried out at λex = 465 nm. Excitation and emission slit widths used were 2.5 nm, and a scan speed of 200 nm/min was used. The fluorescence lifetimes were measured from time-resolved intensity decays by the time-correlated single-photon counting (TCSPC) technique. The light source used was a picosecond diode laser (Nano LED) at 440 nm (Horiba JobinYvon). The fluorescence decays were deconvoluted using Datastation software for acquisition and IBH DAS6 for data analysis. The signals were collected at the magic angle (54.7°). The decay times were determined using the nonlinear least-squares method of the TCSPC technique. The contact angle was measured by Digidrop (contact angle meter model DS). Microscopy Studies. All of the samples for atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) were prepared at 6 × 10−4 M in ethanol, and the solutions were sonicated for 10 min before these were drop-casted and allowed to dry under an IR lamp. Whereas the samples for AFM were prepared on a silicon wafer and SEM on aluminum foil, those for TEM were prepared on a carbon-coated copper grid. Computational Study. All of the calculations for the conjugate of calix[4]arene (L) and its complex with the Hg2+ ion were computed using the Gaussian 09 package.51 The model structure has been prepared by bringing some modifications in the arm portions of a crystal structure reported by us recently.52 Geometry optimization has been performed in a cascade manner to obtain the lowest-energy structure. In the first step, geometry optimization was carried out at the PM6 level of theory, followed by the B3-LYP/631G(d,p) level with zero-point energy correction to get the lowest-energy structure of L. The optimized structure of L thus obtained was used for its 1:1 complexation with Hg2+ by appropriately placing this ion in the vicinity of the binding core. The complex was optimized at the same level of theory with the application of the SDD basis set and effective core potential on the transition metal ion, Hg2+. The interaction energy was computed by subtracting the single-point energy of each of the individual components from the total energy of the 1:1 complex, all being treated at the same level of theory. As the peak in the Job plot is broad enough to fit with either the 1:1 or 1:2 complex, even the 1:2 complex of Hg2+ vs L was optimized using two-layered ONIOM calculations.53 In this, the Hg2+ ion and the NBD units of both the calixarene molecules are treated at the B3LYP/6-31G(d,p)/SDD(Hg2+) level of theory, whereas the rest is treated under the 16996
DOI: 10.1021/acsomega.8b02848 ACS Omega 2018, 3, 16989−16999
ACS Omega
Article
semiempirical PM6 method. The regions treated under ONIOM and DFT can be understood from the optimized structure given in the Supporting Information (SI18, Figure S16). For the BSSE-corrected interaction energy calculations, the obtained geometry of the 1:2 complex from the two-layer ONIOM computations was subjected to single-point energy calculations at the B3LYP/6-31G(d,p)/SDD(Hg2+) level. Synthesis and Characterization of L. The steps involved in the synthesis of L are given in Scheme 1. As the precursors P1 to P5 were already reported by us,54 their characterization data are given in the Supporting Information (SI01). To a solution of 7-chloro 4-nitro benzofurazan (0.2 g, 2.2 mmol) in dry ethanol, P5 was added (0.95 g, 1.0 mmol) along with a catalytic amount (0.1 mL) of pyridine. The reaction was stirred at room temperature for 1 day. The solvent was removed under reduced pressure, and the residue was purified by silica gel column chromatography using ethylacetate: pet ether in 80:20 as eluting medium. Yield: 70%. ESI-MS: HRMS peak for [M + H]+ (m/z) for C64H74N10O12S2 = 1277.4557 (calcd) and 1277.456 (obtained). 1H NMR (400 MHz, CDCl3): δ 0.92 (s, 18H, −C(CH3)3), 1.9 (quint., 4H, CH2CH2SCH3), 2.31 (m, 6H SCH3, CH2CH2CH2N), 2.61 (t, 4H, CH2CH2N), 3.83 (t, 4H, NHCH2CH2), 3.37 (d, J = 13.0 Hz, 4 H, ArCH2Ar), 3.61 (m, 4H, NHCH2CH2CH2S), 3.89 (t, 4 H, OCH2), 4.11 (d, J = 13.0 Hz, 4H, ArCH2Ar), 4.78 (t, 2H, CONH), 6.21 (NBDArH), 6.48 (t, 2H, CONH), 6.79 (s, 4 H, ArH), 7.59 (s, 4 H, ArH), 8.29 (NBD-ArH). 13C NMR: 155.33, 147.85, 146.20, 145.91, 145.12, 144.81, 144.58, 143.71, 138.51, 138.11, 136.64, 128.62, 127.31, 120.93, 106.32, 103.17, 79.82, 58.66, 51.20, 46.1, 34.11, 34.52, 31.74, 31.47, 29.46, 29.15, 24.82, 24.97, 22.54, 14.47, 8.83. Synthesis of AuNPcit. Citrate-capped gold nanoparticles were synthesized using the procedure reported in the literature.55,56 Aqueous solution of 0.25 mM HAuCl4 (100 mL) was heated in a conical flask. To the boiling solution, 3 mL of aqueous solution of 1% (w/w) sodium citrate was added. Heating was continued approximately for 15 min until a ruby red color appeared, which indicates the formation of gold nanoparticles (AuNPcit). The resulting AuNPcit’s were centrifuged at 6000 rpm for 15 min to separate them from excess citrate. The solution pH was adjusted to 7.4 with 0.1 M NaOH. These AuNPcit were characterized by TEM and UV− vis spectroscopy techniques. Synthesis of AuNPL. Aqueous solution of AuNPcit (2 mL) was centrifuged at 8000 rpm, and the settled nanoparticles were taken and dispersed in ethanol (2 mL). The receptor L (10 mmol) was dissolved in a minimum amount of THF (0.2 mL) and was diluted with 1.8 mL of ethanol. The two solutions were mixed and stirred for ∼2 h. The resulting solution was centrifuged at 7000 rpm, followed by dispersion in ethanol to remove the L present in excess, and washed with ethanol several times until the wash liquor became colorless. The resulting NPs were capped by the calixarene conjugate (AuNPL), and these were dispersed in ethanol for further use.
■
related spectra and the corresponding data, computation-related data, etc. (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Chebrolu Pulla Rao: 0000-0002-1004-0028 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS CPR acknowledges financial support from the DST/SERB {EMR/2014/000985} and J. C. Bose National Fellowship {SB/S2/JCB-066/2015} and IIT Bombay for Institute Chair Professorship. B.U. acknowledges CSIR for the award of Senior Research Fellowship {09/087(0835)/2015-EMR-I}, and S.J. acknowledges SERB for National Post-Doctoral Fellowship {PDF/2017/000236}. We thank Aekta Upadhyay and Rahul Nag for some help with SEM and TEM, respectively. We acknowledge SAIF and central facilities of IIT Bombay, including AFM, SEM, TEM, XPS, and contact angle measurement.
■
REFERENCES
(1) Kim, K. H.; Kabir, E.; Jahan, S. A. A review on the distribution of Hg in the environment and its human health impacts. J. Hazard. Mater. 2016, 306, 376−385. (2) Boening, D. W. Ecological effects, transport, and fate of mercury: a general review. Chemosphere 2000, 40, 1335−1351. (3) Wester, P. W. Histopathological effects of environmental pollutants β-HCH and methyl mercury on reproductive organs in freshwater fish. Comp. Biochem. Physiol., C: Comp. Pharmacol. 1991, 100, 237−239. (4) Eagles-Smith, C. E.; Wiener, J. G.; Eckley, C. S.; Willacker, J. J.; Evers, D. C.; DiPasquale, M. M.; Obrist, D.; Fleck, J. A.; Aiken, G. R.; Lepak, M.; Jackson, A. K.; Webster, J. P.; Stewart, A. R.; Davis, J. A.; Alpers, C. N.; Ackerman. Mercury in western North America: A synthesis of environmental contamination, fluxes, bioaccumulation, and risk to fish and wildlife. Sci. Total Environ. 2016, 568, 1213−1226. (5) Scheuhammer, A.; Braune, B.; Chan, H. M.; Frouin, H.; Krey, A.; Letcher, R.; Loseto, L.; Noel, M.; Ostertag, S.; Ross, P.; Wayland, M. Recent progress on our understanding of the biological effects of mercury in fish and wildlife in the Canadian arctic. Sci. Total Environ. 2015, 509−510, 91−103. (6) De Flora, S.; Bennicelli, C.; Bagnasco, M. Genotoxicity of mercury compounds. A review. Mutat. Res. 1994, 317, 57−79. (7) Crespo-López, M. E.; Macedoa, G. L.; Pereira, S. I. D.; Arrifanoa, G. P. F.; Dinizc, D. L. W. P.; Nascimento, J. L. M. D.; Herculanoc, A. M. Mercury and human genotoxicity: critical considerations and possible molecular mechanisms. Pharmacol. Res. 2009, 60, 212−220. (8) Kim, K. H.; Kabir, E.; Jahan, S. A. A review on the distribution of Hg in the environment and its human health impacts. J. Hazardous Mater. 2016, 306, 376−385. (9) Leray, I.; Valeur, B. Calixarene-based fluorescent molecular sensors for toxic metals. Eur. J. Inorg. Chem. 2009, 3525−3535. (10) Kim, J. S.; Quang, D. T. Calixarene-derived fluorescent probes. Chem. Rev. 2007, 107, 3780−3799. (11) Kim, H. N.; Ren, W. X.; Kim, J. S.; Yoon, J. Fluorescent and colorimetric sensors for detection of lead, cadmium and mercury ions. Chem. Soc. Rev. 2012, 41, 3210−3244. (12) Ludwig, R.; Dzung, N. T. K. Calixarene based molecule for cation recognition. Sensors 2002, 2, 397−416.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02848. Spectral data for the characterization of all of the precursors and the final molecules, all of the titration16997
DOI: 10.1021/acsomega.8b02848 ACS Omega 2018, 3, 16989−16999
ACS Omega
Article
(13) Joseph, R.; Rao, C. P. Ion and molecular recognition by lower rim 1,3-di-conjugates of calix[4]arene a receptors. Chem. Rev. 2011, 111, 4658−4702. (14) Rurack, K.; Genger, U. T. Rigidization, preorientation and electronic decoupling- the magic triangle’ for the design of highly efficient fluorescent sensors and switches. Chem. Soc. Rev. 2002, 31, 116−127. (15) Caballero, A.; Martinez, R.; Lloveras, V.; Ratera, I.; Gancedo, J. V.; Wurst, K.; Tarraga, A.; Molina, P.; Veciana, J. Highly selective chromogenic and redox or fluorescent sensors for Hg2+ in aqueous environment based on 1, 4- disubstitutedazines. J. Am. Chem. Soc. 2005, 127, 15666−15667. (16) Carter, K. P.; Young, A. M.; Palmer, A. E. Fluorescent sensors for measuring metal ions in living systems. Chem. Rev. 2014, 114, 4564−4601. (17) Nolan, E. M.; Lippard, S. J. Turn on and ratiometric mercury sensing in water with red-emitting probe. J. Am. Chem. Soc. 2007, 129, 5910−5918. (18) Aksuner, N.; Basaran, B.; Henden, E.; Yilmaz, I.; Cukurovali, A. A sensitive and selective fluorescent sensor for the determination of mercury (II) based on a novel triazine-thione derivative. Dyes Pigm. 2011, 88, 143−148. (19) Ding, Y.; Wang, S.; Li, J.; Chen, L. Nanomaterial-based optical sensors for mercury ions. TrAC, Trends Anal. Chem. 2016, 82, 175− 190. (20) McNicholas, T. P.; Zaho, K.; Yang, C.; Hernandez, S. C.; Mulchandani, A.; Myung, N. V.; Deshusses, M. A. Selective detection of elemental mercury vapor by gold-nanoparticle-decorated carbon nanotube sensors. J. Phys. Chem. C 2011, 115, 13927−13931. (21) Darbha, G. K.; Ray, A.; Ray, P. C. Gold nanoparticle-based miniaturized nanomaterial surface energy transfer probe for rapid and ultrasensitive detection of mercury in soil, water and fish. ACS Nano 2007, 1, 208−214. (22) Lee, H.; Lee, H.-S.; Reibenspies, J. H.; Hancock, R. D. Mechanism of “Turn-on’’ fluorescent sensors for mercury(II) in solution and its implication for ligand design. Inorg. Chem. 2012, 51, 10904−10915. (23) Lee, H.; Lee, S. S. Thiaoxaaza- macrocyclic chromoionophores as mercury (II) sensors: synthesis and colour modulation. Org. Lett. 2009, 11, 1393−1396. (24) Lu, J.; Tong, X.; He, X. A mercury ion-selective electrode based on a calixarene derivative containing the thiazole azo group. J. Electroanal. Chem. 2003, 540, 111−117. (25) Areti, S.; Yarramala, D. S.; Hinge, V. K.; Khedkar, J.; Rao, C. P.; et al. Glyco-conjugate as selective switch on molecule for Hg2+ in the presence of albumin proteins, blood serum milieu and on silica gel solid support. RSC Adv. 2014, 4, 16290. (26) Areti, S.; Hinge, V. K.; Rao, C. P. Pyrenyl-imino-C2-glucosyl conjugate: synthesis, characterization, and ratiometric and reversible ON-OFF receptor for Hg(2+). Carbohydr. Res. 2014, 399, 64−69. (27) Métivier, R.; Leray, I.; Lebeau, B.; Valeur, B. A mesoporous silica functionalised by a covalently bound calixarene fluoroionophore for selective optical sensing of mercury (II) in water. J. Mater. Chem. 2005, 15, 2965−2973. (28) Talanova, G. G.; Elkarim, N. S. A.; Talanov, V. S.; Bartsch, R. A. A calixarene based fluorogenic reagent for selective mercury (II) recognition. Anal. Chem. 1999, 71, 3106−3109. (29) Chena, Q. Y.; Chna, C. F. A new Hg2+- selective fluorescent sensor based on a dansyl amide-armed calix[4]-aza-crown. Tetrahedron Lett. 2005, 46, 165−168. (30) Lee, Y. H.; Lee, M. H.; Zhang, J. F.; Kim, J. S. Pyrene ExcimerBased Calix[4]arene FRET Chemosensor for Mercury(II). J. Org. Chem. 2010, 75, 7159−7165. (31) Joseph, R.; Ramanujam, B.; Acharya, A.; Rao, C. P. Lower rim 1,3-di{bis(2-picolyl)}amide derivative of calix[4]arene (L) as ratiometric primary sensor toward Ag+ and the complex of Ag+ as secondary Sensor toward Cys: experimental, computational, and microscopy studies and INHIBIT logic gate properties of L. J. Org. Chem. 2009, 74, 8181−8190.
(32) Li, H.; Zhang, Y.; Wang, X.; Xiong, D.; Bai, Y. Calixarene capped quantum dots as luminescent probe for Hg2+ions. Mater. Lett. 2007, 61, 1474−1477. (33) Catrow, J. L.; Zhang, Y.; Zhang, M.; Ji, H. Discovery of selective small-molecule inhibitors for the β-Catenin/T-cell factor proteinprotein interaction through the optimization of the acyl hydrazone moiety. J. Med. Chem. 2015, 58, 4678−4692. (34) Wan, X.; Liu, S. Fluorescent water - soluble responsive polymer site-specifically labelled with FRET dyes possessing pH- and thermomodulated multicolour fluorescence emissions as dual ratiometric probes. J. Mater. Chem. 2011, 21, 10321−10329. (35) Yamaguchi, T.; Asanuma, M.; Nakanishi, S.; Saito, Y.; Okazaki, M.; Dodo, K.; Sodeoka. Turn-On fluorescent affinity labeling using a small bifunctional O-nitrobenzoxadiazole unit. Chem. Sci. 2014, 5, 1021. (36) Wu, S.; Wang, X.; Zhu, C.; Song, Y.; Wang, J.; Li, Y.; Guo, Z. Monofunctional platinum complexes containing a 4-nitrobenzo-2-oxa1, 3-diazole fluorophore: distribution in tumor cells. Dalton Trans. 2011, 40, 10376. (37) Uchiyama, S.; Santa, T.; Imai, K. Study on the fluorescent ‘onoff ’ properties of benzofurazan compounds bearing an aromatic substituent group and design of fluorescent ‘on-off’ derivatization reagents. Analyst 2000, 125, 1839−1845. (38) Zhang, S.; Yang, H.; Ma, Y.; Fang, Y. A fluorescent bis-NBD derivative of calix[4]arene: switchable response to Ag+ and HCHO in solution phase. Sens. Actuators, B 2016, 227, 271−276. (39) Uttam, B.; Kandi, R.; Hussain, M. A.; Rao, C. P. Fluorescent Lower Rim 1,3-Dibenzooxadiazole Conjugate of Calix[4]arene in Selective Sensing of Fluoride in Solution and in Biological Cells Using Confocal Microscopy. J. Org. Chem. 2018, 83, 11850−11859. (40) Consoli, G. M. L.; Granata, G.; Fragassi, G.; Grossi, M.; Sallese, M.; Geraci, M. Design and synthesis of multivalent fluorescent folatecalix[4]arene conjugate: cancer cell penetration and intracellular localization. Org. Biomol. Chem. 2015, 13, 3298. (41) Cormode, D. P.; Evans, A. J.; Davis, J. J.; Beer, P. D. Amplification of anion sensing by disulfide functionalized ferrocene and ferrocene-calixarene receptors absorbed onto gold surfaces. Dalton Trans. 2010, 39, 6532−6541. (42) Valkenier, H.; Malytskyi, V.; Blond, P.; Retout, M.; Mattiuzzi, A.; Goole, J.; Raussens, V.; Jabin, I.; Bruylants, G. Controlled functionalization of gold nanoparticles with mixtures of calix[4]arenes revealed by infrared spectroscopy. Langmuir 2017, 33, 8253−8259. (43) Troian-Gautier, L.; Valkenier, H.; Mattiuzzi, A.; Jabin, I.; Brande, N. V. D.; Mele, B. V.; Hubert, J.; Reniers, F.; Bruylants, G.; Lagrost, C.; Leroux, Y. Extremely robust and post-functinalizable gold nanoparticles coated with calix[4]arenes via metal-carbon bonds. Chem. Commun. 2016, 52, 10493. (44) Luo, C.; Wang, Y.; Li, X.; Jiang, X.; Gao, P.; Sun, K.; Zhou, J.; Zhang, Z.; Jiang, Q. An optical sensor with polyaniline-gold hybrid nanostructures for monitoring pH in saliva. Nanomaterials 2017, 7, 67. (45) Fery-Forgues, S.; Fayetb, P. J.; Lopez, A. Drastic changes in the fluorescence of NBD probes with the polarity of the medium: involvement of TICT state. J. Photochem. Photobiol., A 1993, 70, 229− 243. (46) Reichardt, C. Solvatochromic dyes as solvent polarity indicators. Chem. Rev. 1994, 94, 2319−2358. (47) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 3rd ed.; Wiley-VCH: Weinheim, 2003. (48) Nehra, A.; Yarramala, D. S.; Hinge, V. K.; Samanta, K.; Rao, C. P. Differentiating phosphates by an Mg2+ complex of the conjugate of calix[4]arene via the formation of ternary species and causing changes in the aggregation: spectroscopy, microscopy and computational modeling. Anal. Chem. 2015, 87, 9344−9351. (49) Mummidivarapu, V. V. S.; Bandru, S.; Yarramala, D. S.; Samanta, K.; Mhatre, D. S.; Rao, C. P. Binding and ratiometric dual ion recognition of Zn2+ and Cu2+ by 1, 3, 5-tris-amidoquinoline conjugate of calix[6]arene by spectroscopy and its supramolecular features by microscopy. Anal. Chem. 2015, 87, 4988−4995. 16998
DOI: 10.1021/acsomega.8b02848 ACS Omega 2018, 3, 16989−16999
ACS Omega
Article
(50) Joseph, Y.; Besnard, I.; Rosenberger, M.; Guse, B.; Nothofer, H. G.; Wessels, J. M.; Wild, U.; Gericke, A. K.; Su, D.; Schlogl, R.; Yasuda, A.; Vossemeyer, T. Self-assembled gold nanoparticle/ alkanedithiol films: preparation, electron microscopy, XPS-analysis, charge transport and vapor-sensing properties. J. Phys. Chem. B. 2003, 107, 7406−7413. (51) Frisch, M. J.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Norm, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09; Gaussian, Inc.: Wallingford, CT, 2009. (52) Bandela, A.; Chinta, J. P.; Rao, C. P. Role of conformational changes brought in the arms of the 1,3-di-capped conjugate of calix[4]arene (L) in turning of fluorescence of L by Hg2+. Dalton Trans. 2011, 40, 11367−11370. (53) Dapprich, S.; Komáromi, I.; Byun, K. S.; Morokuma, K.; Frisch, M. J. A New ONIOM Implementation in Gaussian 98. 1. The Calculation of Energies, Gradients and Vibrational Frequencies and Electric Field Derivatives. J. Mol. Struct.: THEOCHEM 1999, 461− 462, 1−21. (54) Samanta, K.; Rao, C. P. A bifunctional thioether linked coumarin appended calyx[4]arene acquires selectivity towards Cu2+ sensing on going from solution to SAM on gold. ACS Appl. Mater. Interfaces 2016, 8, 3135−3142. (55) Nghiem, T. H. L.; La, T. H.; Vu, X. H.; Chu, V. H.; Nguyen, T. H.; Le, Q. U.; Fort, E.; Do, Q. H.; Tran, H. N. Synthesis, capping and binding of colloidal gold nanoparticles to proteins. Adv. Nat. Sci.: Nanosci. Nanotechnol. 2010, 1, No. 025009. (56) Alkilany, A. M.; Abulateefeh, S. R.; Mills, K. K.; Yaseen, A. I. B.; Hamaly, M. A.; Alkhatib, H. S.; Aiedeh, K. M.; Stone, J. W. Colloidal stability of citrate and mercaptoacetic acid capped gold nanoparticles upon lyophilisation: effect of capping ligand attachement and types of cryoprotectants. Langmuir 2014, 30, 13799−13808.
16999
DOI: 10.1021/acsomega.8b02848 ACS Omega 2018, 3, 16989−16999
