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Article Cite This: ACS Omega 2018, 3, 17220−17226
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Few Particle-Level Chromaticity Index-Based Discrimination of Biothiols Using Chemically Interactive Dual-Emitting Nanoprobe Chirantan Gayen,† Srestha Basu,† Uday Narayan Pan,‡ and Anumita Paul*,† †
Department of Chemistry and ‡Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati 781039, India
ACS Omega 2018.3:17220-17226. Downloaded from pubs.acs.org by 46.161.56.115 on 12/18/18. For personal use only.
S Supporting Information *
ABSTRACT: Herein, we introduce a unique approach for discrimination between two important biothiols (BTs): cysteine (Cys) and glutathione (Glu), on the basis of changes in chromaticity coordinates of a dual-emitting “a few particle” nanoprobe. A novel dual-channel emission nanocomposite was developed by zinc-mediated conjugation of green-emitting fluorescein with red-emitting gold nanoclusters (Au NCs), hereafter referred to as Zn−FL−Au NCs. Cys and Glu having different interactions with Zn−FL−Au NCs were found to have different effects on the overall luminescence of Zn−FL−Au NCs, which eventually led to observation of different chromaticity indices of the latter upon interaction with these two BTs. For example, upon addition of Cys to a dispersion of Zn−FL−Au NCs having chromaticity coordinates (0.38, 0.49), the luminescence peak due to Au NCs was found to have been selectively quenched, as a consequence of which the chromaticity coordinates of Zn−FL−Au NCs changed to (0.33, 0.52). On the other hand, upon interaction with Glu, the intensity of the luminescence peak due to Au clusters as well as fluorescein (FL) enhanced to a certain extent, however, keeping the chromaticity coordinates almost similar to those of Zn−FL−Au NCs (0.38, 0.47). Allied results were obtained from super-resolution microscopic analysis wherein luminescence of “a few particle” of Zn−FL−Au NCs was used to probe the differential interaction with the aforementioned biothiols, leading to a different degree of changes in the Commission Internationale d’É clairage (CIE) coordinates of the former. Also, the specificity of the nanoprobe toward Cys was confirmed by monitoring the interactions of Zn−FL−Au NCs with other interfering chemical compounds. Moreover, the optical properties of Zn−FL−Au NCs were found to be retained following incubation in human blood serum, which further supports the use of this nanoprobe for discrimination of BTs in practical systems.
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INTRODUCTION The surge in research endeavors toward development of luminescence-based sensors has enabled facile and sensitive detection of biologically relevant molecules,1 nitro explosives,2 volatile organic compounds,3 and a variety of metal ions,4,5 among others. Notwithstanding the notable progress made in this regard, conventional molecular sensors are often associated with inherent limitations, which eventually restrict their wide range application potential. For example, selectivity of a sensor toward a particular analyte in preference to its chemical analogues is an important premise of concern, requiring urgent attention. The origin of this limitation possibly arises from the single-channel emitting nature of the sensors. This is because many factors other than the analyte may also affect the single-channel emission of the sensor due to which the specificity of a sensor toward an analyte is often lost. In addition, a plethora of “turn-off” and “turn-on” sensors have been developed on the basis of the principles of “aggregation induced emission” and “aggregation caused quenching”.6−11 However, several limitations to such strategies may originate from the interference of solvents, similar chemical structure, and allied acidity (and basicity) of the analytes. A probable way out of the aforementioned restrictions may emerge from the principle of chemical interactions between © 2018 American Chemical Society
multicomponent (hence multichromatic) sensors and analytes causing differential response in the components of the sensor. In this regard, monitoring the changes in luminescence of a single particle of a multichromic sensor in presence of an analyte portends to offer a greater degree of precision in the detection process. This is because in most of the sensing experiments performed using an ensemble of molecules, an “averaging of properties” is observed, which not only restricts the detection limit but also obscures important information related to the fate of a single analyte molecule. Thus, designing newer principles of sensing by combination of both these options, i.e., “a few particle”-level luminescence change in a multichromic luminophore, may offer the best of both choices.12 This becomes even more important when the challenge of discriminating between chemically analogous compounds is at hand. It is noteworthy that in such cases, a distinction between two analogous analytes is mainly done on the basis of the “extent” of a similar interaction with the sensor,13 which may not necessarily offer the best degree of precision in the detection process. Further, monitoring the Received: September 13, 2018 Accepted: November 30, 2018 Published: December 14, 2018 17220
DOI: 10.1021/acsomega.8b02373 ACS Omega 2018, 3, 17220−17226
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was used for all experiments. All chemicals were used as received without further purification. Synthesis of Gold Nanoclusters (Au NCs). Gold nanoclusters were synthesized following slight modification of an earlier report.22 Briefly, 0.4 mL of HAuCl4 (10 mM) was added to 10 mL of water. Separately, ∼5 mg of 11mercaptoundecanoic acid (MUA) was dissolved in 1 mL of water using 0.2 mL of sodium hydroxide (1 M). One milliliter of this solution was added to the solution of HAuCl4 mentioned above. The resultant solution was stirred for 4 h at room temperature. This led to formation of a red luminescent dispersion (λemission = 610 nm) upon excitation at 280 nm. Synthesis of Zinc Fluoresceinate. Fluorescein (∼1.5 mg) was added to water (4 mL). This led to partial dissolution of fluorescein. Fluorescein (2 mL) solution was taken and added with zinc acetate dihydrate (∼150 mg). The resultant solution was stirred for a while and finally added to 0.2 mL of NaOH (1 M). This led to formation of a yellow dispersion. The resultant dispersion was centrifuged at a speed of 10 000 rpm for 5 min. The pellet obtained was further washed with water, and the cycle was repeated further. The supernatant was discarded, and the pellet following redispersion in water was used for further experiments. Synthesis of Composite Comprising of Zinc Fluoresceinate and Au NCs (Zn−FL−Au NCs). The pellet of zinc fluoresceinate as obtained above was dissolved in 1 mL of water and added to 5 mL of Au NCs. The dispersion was stirred for 15 min, followed by centrifugation at a speed of 10 000 rpm for 5 min. The supernatant was discarded and the pellet was washed with water. This cycle was repeated four times to ensure complete removal of the starting materials. Finally, the pellet was redispersed in water and used for further experiments. Interaction of Zn-Fl-Au NCs with Cys and Glutathione. The obtained pellet of Zn−FL−Au NCs (as mentioned above) was dissolved in 1 mL of water. This dispersion (0.2 mL) was added to 0.8 mL of water to make a 1 mL dispersion of the composite. Further, 0.2 mL of the diluted dispersion (from 1 mL above) was added to 2 mL of phosphate buffer (pH = 7.04). The aforementioned dispersion was added with 0.05 mL of Cys and Glu to study the differential interaction of Zn-Fl-Au NCs with Cys and Glu. Details of Confocal Laser Scanning Microscopic (CLSM) Analysis. Confocal measurements of all samples were carried out using a Carl Zeiss LSM-880 confocal laser scanning microscope. All samples were excited at 355 nm using a maximum laser power of 30 mW. All analysis was done using a GaAsP and Airyscan detector. Images were captured at a frame size of 64 × 64 μm2. The resultant data were analyzed by ZEN black software. The spectra of Zn−FL−Au NCs were acquired using lambda scan function of the CLSM.
bulk properties of a luminophore in presence of analytes may lead to loss of relevant information as regards one-to-one interaction between the sensor and the analyte. Thus, demonstration of “a few particle”-level distinction among compounds with allied functional groups using multichromic luminophores is deemed important. In this regard, luminescence-based distinction of biothiols (BTs) is an important challenge worthy of pursuit.14−18 As per research reports, most attempts for discrimination of relevant biothiols have been based on reaction of the thiol group of a BT (Cys, homo Cys, and glutathione) with strategically designed fluorophores, thereby causing changes in the fluorescence of the latter.19,20 Other effective methods include aromatic substitution and rearrangement reactions in presence of these BTs.21 However, as mentioned above, such techniques primarily offer discrimination between analytes mainly on the basis of the “extent of change” in fluorescence of the sensor. Also, as per literature reports, a majority of the studies for luminescence-based discrimination of BTs have been performed in bulk phase,14−21 as a consequence of which, the maximum degree of accuracy in detection of BTs is yet to be realized. Hence, to develop an unambiguous and precise strategy for discrimination of BTs, “a few particle”-level analysis of a multichromic luminophore undergoing different chemical interactions with the BTs is of considerable importance worthy of pursuit. Herein, we report the use of a dual-emitting nanoprobe (Zn−FL−Au NCs) comprising of red-emitting gold nanoclusters (Au NCs) conjugated with green-emitting fluorescein (FL) via zinc ions, for “a few particle”-level discrimination of Cys and Glu by super-resolution microscopic analysis. In solution phase, the red luminescence due to Au NCs (in Zn− FL−Au NCs) was found to get quenched in presence of Cys, whereas the green luminescence due to FL was found to be enhanced. This led to considerable change in the chromaticity coordinates of as-prepared Zn−FL−Au NCs. On the other hand, upon interaction with Glu, enhancement in luminescence of both Au NCs and FL occurred, which, as a result, did not affect the intrinsic chromaticity coordinates of Zn−FL−Au NCs. The luminescent results obtained at “a few particle” level were found to be concordant with those of the bulk solution phase results. A possible mechanism for differential interaction of Zn−FL−Au NCs with Cys and Glu has been attributed to ligand replacement reaction between Zn−FL−Au NCs and Cys on one hand and nonfeasibility of such a reaction between Zn−FL−Au NCs and Glu due to steric crowding on the other. Thus, the uniqueness of our study lies in the following facts: first, to the best of our knowledge, this is the first report on “a few particle”-level interaction-based distinction of BTs. Second, we introduce a novel concept of discrimination between analytes using a dual-emitting nanoprobe with consequential changes in the chromaticity coordinates of the latter. Finally, the concept of specific chemical interaction of an analyte with one component of a multicomponent emitter may circumvent the persistent issue of “lack of selectivity and sensitivity” of a sensor toward an analyte.
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RESULTS AND DISCUSSION Addition of mercaptoundecanoic acid (MUA) to a solution of HAuCl4 led to the formation of a colorless solution, which upon excitation at 280 nm was luminescent with an emission maxima at ∼610 nm (Figure S1A), suggesting the formation of Au nanoclusters (Au NCs). Transmission electron microscopic analysis confirmed the presence of particles less than 2 nm, which affirmed the formation of Au NCs (Figure S1B). These red-emitting Au NCs were conjugated with green emitting fluorescein (FL) by coordination with zinc ions. Addition of
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EXPERIMENTAL SECTION Materials and Methods. Tetrachloroauric acid (HAuCl4), fluorescein, and 11-mercaptoundecanoic acid (MUA) were procured from Sigma-Aldrich. Zinc acetate dihydrate and sodium hydroxide were purchased from Merck. Milli-Q water 17221
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Figure 1. (A) CLSM images of “a few particle” emitters of Zn−FL−Au NCs. (B) Photoluminescence (PL) spectrum acquired on “a few particle” emitter shown in (A) and (C) corresponding chromaticity coordinates. (D) CLSM images of “a few particle” emitter of Zn−FL−Au NCs treated with Cys. (E) PL spectrum acquired on “a few particle” emitter shown in (D) and (F) corresponding chromaticity coordinates. (G) CLSM images of “a few particle” emitter of Zn−FL−Au NCs treated with Glu. (H) PL spectrum acquired on a “a few particle” emitter shown in (G) and (I) corresponding chromaticity coordinates. Concentrations of Cys and Glu were maintained at 2.6 mM.
presence of emission maxima at ∼520 and ∼610 nm attributed to emission of FL and Au NCs, respectively (Figure S7A−C). Moreover, the XRD profile of Zn-FL was significantly altered upon formation of Zn−FL−Au NCs, thereby indicating structural alteration of Zn-FL owing to interaction with Au NCs (Figure S8A). Scanning transmission electron microscopic analysis revealed the formation of featureless Zn−FL− Au NCs moieties. Further, elemental mapping analysis was performed, which showed that the Au NCs could be decorated on zinc fluoresceinate (Figure S8B−F). To quantitate the amount of zinc and gold ions present in the composite, inductively coupled plasma atomic emission spectrometric (ICP-AES) analysis was performed. It was found that 178 ppm zinc and 17.7 ppm gold (in 5.5 mL of HNO3) were present in Zn−FL−Au NCs. The as-prepared composite was further used for “a few particle”-level discrimination of BTs by a confocal laser scanning microscope (CLSM). Typically, a dilute solution of the composite, drop cast on a microscope slide, was analyzed under CLSM and “a few particle” emitter was identified. To substantiate the few particle emitting nature of the composite, fluorescence intermittency (commonly known as blinking) profile of the “a few particle” emitter was acquired (Figure S9A). It was interesting to observe that the blinking profile comprised of square wave “off” periods and the photobleaching occurred by a sudden drop to the background level instead of a gradual decrease in luminescence intensity, thereby indicating
zinc acetate dihydrate solution to fluorescein (FL) led to enhancement in fluorescence of FL (Figure S2). Electrospray ionization mass spectrometric (ESI-MS) analysis revealed the formation of a 1:1 complex possibly bonded through Zn2+ ion to the carboxylate group of FL at pH = 8 (Figure S3). Powder X-ray diffraction (XRD) characterization of Zn-FL indicated generation of new peaks unmatched with those of the precursors, thereby indicating possible incorporation of zinc ion in the lattice of FL (Figure S4). On the other hand, given the feasibility of coordination of zinc ions with the carboxylate groups of MUA attached to the surface of Au NCs, conjugation of FL with Au NCs via zinc ions seemed a facile strategy for formation of Zn−FL−Au NCs. It was interesting to observe that upon addition of increasing amount of zinc ions to Au NCs solution, the luminescence intensity of Au NCs underwent gradual enhancement, indicating aggregation of Au NCs in presence of zinc ions via possible coordination (Figure S5).23,24 Intriguingly, addition of Au NCs to Zn-FL resulted in formation of a composite (Zn−FL−Au NCs) with two distinct absorbance maxima at ∼400 and ∼498 nm, attributed to absorbance due to Au NCs and FL, respectively (Figure S6A− C). Moreover, as the absorbance peaks were observed to be present following repeated centrifugation and redispersion of the composite, it implied possible attachment of Au NCs to FL via zinc ions. In an allied vein, upon 355 nm excitation, the luminescence spectrum of Zn−FL−Au NCs showed the 17222
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Figure 2. (A) PL spectra of a dispersion of Zn-Fl-Au NCs. (B) PL spectra of (a) Zn-Fl-Au NCs and (b) Zn-Fl-Au NCs treated with 50 μL of Cys (∼165 mM). (C) PL spectra of (a) Zn-Fl-Au NCs and (b) Zn-Fl-Au NCs treated with 50 μL of Glu (∼165 mM). (D) Chromaticity coordinates of (a) Zn-Fl-Au NCs and those following addition of (b) Cys and (c) Glu.
from FL and Au NCs components, respectively (λexc = 355 nm) (Figure 2A). It is to be mentioned here that the emission profile of the composite in the solution phase differed slightly from that in the solid phase (containing same proportion of Au NCs and FL) where the luminescence due to Au NCs was observed to be relatively much lower in solution phase as compared to that in the solid state. This could be attributed to the well-known “restricted intramolecular motion (RIM)”enhanced fluorescence of Au NC peak in solid state of Zn− FL−Au NCs. It was interesting to observe that although the luminescence due to Au NCs was effectively quenched upon addition of (50 μL of ∼165 mM) Cys to the dispersion containing Zn−FL−Au NCs (Figure 2B), addition of (50 μL of ∼165 mM) Glu to Zn−FL−Au NC dispersion led to enhanced luminescence of Au NCs and to an extent enhanced luminescence of FL as well (Figure 2C). As a result, for the solution phase, the calculated chromaticity coordinates of Zn−FL−Au NCs were found to be (0.38, 0.49), which upon addition of Cys changed to (0.33, 0.52), (owing to significant quenching of the luminescence due to Au NCs), whereas on the other hand, upon addition of Glu, the chromaticity coordinates changed to (0.38, 0.47) (owing to simultaneous enhancement in emission due to FL and Au NCs) (Figure 2D). Discernible discrimination between the two thiols could be achieved in a concentration as low as 530 μM in solution phase (Figure S11). However, it should be mentioned here that at a single (or a few-particle) level in principle, discrimination at even lower concentrations (of the order of a few molecules) should be achieved, which is rather unprecedented in the current literature. In this regard, we have carried out the discrimination of biothiols at “a few particle” level and higher sensitivity of the nanoprobe could be achieved therein. Interestingly, upon treatment of Zn−FL−Au NCs with Cys and Glu and recording the corresponding spectra at “a few particle” level, sensitivity as high as 2.6 nM could be achieved (additional Figure S22). This further substantiates the
the few particle emitting nature of the particle. Further, to localize the positions of the “few particle” nanoprobes of the composite, the photon spatial distribution was measured and fitted with a point spread function (PSF) (Figure S9B). Given the low density and high purity of emitters used in analysis, each isolated PSF corresponded to the image of “a few particle” emitter. Thereafter, the centre of the obtained PSF was determined using PSF fitting algorithms, which resolved the position of the “a few particle” emitters. To this end, the luminescence spectrum of such a typical “a few particle” was acquired (Figure 1A,B), which featured two distinct emission peaks at ∼520 and ∼610 nm (λexc = 355 nm) owing to emission of FL and Au NCs, respectively, from “a few particle”. The chromaticity coordinates calculated from the spectrum acquired on such “a few particle” spectrum of Zn−FL−Au NCs were determined to be (0.52, 0.44) (Figure 1C). Similarly, a typical “a few particle” (Figure S10A,B) emitter of the composite treated with Cys was also analyzed under CLSM. Interestingly, the fluorescence spectrum acquired on such a particle exhibited a single emission peak at ∼510 nm owing to emission of FL. However, the spectrum was nearly devoid of the emission of Au NCs, thereby suggesting possible quenching of the cluster emission in presence of Cys. This led to significant alteration of the chromaticity coordinates of Zn− FL−Au NCs to (0.19, 0.23) upon interaction with Cys (Figure 1D−F). On the other hand, super-resolution microscopic analysis revealed that a typical “a few particle” emitter of Zn− FL−Au NCs (Figure S10C,D) treated with Glu exhibited two distinct emission peaks at ∼520 and ∼610 nm (λexc = 355 nm) and the chromaticity coordinates of the emitter calculated from the spectrum therein were found to be (0.40, 0.33) (Figure 1G−I). Allied results were obtained when bulk phase discrimination of Cys and Glu was pursued in presence of Zn−FL−Au NCs. The luminescent dispersion of Zn−FL−Au NCs exhibited two discernible peaks at ∼520 and ∼610 nm owing to emission 17223
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advantage of “a few particle”-level discrimination of analytes over solution phase detection, especially when the latter is associated with low sensitivity in the discrimination process. The practical utility of a chemical sensor is mainly determined by two factors, stability and specificity. Thus, to substantiate the stability of Zn−FL−Au NCs, the same were incubated in human blood serum for 20 h. Interestingly, although the luminescence intensity of Zn−FL−Au NCs was attenuated to a slight extent, the relative intensity of the emission due to FL and Au NCs in Zn−FL−Au NCs was found to remain unaltered (Figure 3A). It is to be mentioned
the chromaticity coordinates of Zn−FL−Au NCs (Figure S13) in blood serum. To justify the critical role of a dual emitting nanoprobe for discrimination of BTs, the effect of Cys and Glu on luminescence of FL (with added zinc ions) and Au NCs, as separate entities, was studied. Interestingly, addition of Cys and Glu had negligible effect on the luminescence of FL (Figure S14A). On the other hand, both Glu and Cys were observed to quench the luminescence of Au NCs by an insignificant amount albeit to a much similar extent (Figure S14B). Thus, as individual entities, neither FL nor Au NCs could enable discrimination of Cys and Glu. However, upon conjugation of the two (FL and Au NCs) by zinc ions, facile discrimination of Glu and Cys was possible. It is worth mentioning here that although the emission maxima of Zn− FL−Au NCs remains same upon interaction with Glu and Cys, the relative intensity of the two emission peaks of Zn−FL−Au NCs at 520 nm (owing to emission from FL) and 610 nm (owing to emission from Au NCs) changes significantly upon interaction with Cys. Precisely, upon interaction between Zn− FL−Au NCs and Cys, the intensity of the emission peak at 610 nm is quenched and the intensity of the emission peak at 520 nm in enhanced, which significantly alters the chromaticity coordinates of emission from Zn−FL−Au NCs, the overall emission of Zn−FL−Au NCs being shifted to green region as compared with the emission of original Zn−FL−Au NCs. However, upon interaction between Zn−FL−Au NCs and Glu, the intensity of the peak at 520 nm remains unaltered and the intensity of the peak at 610 nm is increased, albeit to a small extent. Thus, there are significantly different effects of Cys and Glu on the chromaticity coordinates of Zn−FL−Au NCs. Thus, the method introduced herein portends to form a facile basis of distinction between biothiols using chromaticity coordinates of emission of a fluorophore. Further, in future, sensing devices may be constructed using this principle, which would be coupled with electronic devices capable of computing chromaticity coordinates and further be used for accurate distinction of chemically allied compounds. Although deciphering the detailed mechanism controlling the differential interaction of Cys and Glu with Zn−FL−Au NCs requires further analysis and is beyond the scope of the current study, a plausible mechanism of discrimination of BTs is proposed herein. Zinc ions conjugating the FL component with Au NCs may bind with Cys, thereby displacing the Au NCs present in Zn−FL−Au NCs. In this regard, it may be mentioned here that binding of Cys with zinc ions is a wellknown and facile complexation reaction. Also, as mentioned above, the luminescence of Au NCs enhances upon addition of zinc ions, possibly due to aggregation, leading to restricted intramolecular motion (RIM) of the ligands stabilizing the clusters23,24 (Figure S5). Thus, upon being detached from zinc ions, the Au NCs get segregated and consequently their luminescence is reduced. On the other hand, Glu, owing to its larger size as compared to that of Cys, imposes significant steric crowding upon interaction with zinc ions. Therefore, the nature of interaction of Glu with Zn−FL−Au NCs might be more of noncovalent interactions rather than strong chemical interaction. This might have eventually led to further association of Au NCs and FL, possibly through hydrogen bonding, thereby enhancing both their luminescence. It may be mentioned here that hydrogen bonding in Glu moieties is known to be favorable. To establish the proposed mechanism, further experiments were performed where cystine, which is an
Figure 3. (A) Luminescence spectrum of (a) Zn−FL−Au NCs and (b) those following incubation in human blood serum for 20 h. (B) CIE coordinates of (a) Zn−FL−Au NCs and those following interaction with (b) glucose, (c) lactose, (d) histidine, (e) glycine, (f) aspartic acid, (g) glutamic acid, and (h) NaCl in buffer medium. Concentration of all species was maintained at 2.6 mM.
here that owing to the presence of proteins and other biomolecules in blood serum, an emission peak was observed at ∼425 nm.25 On the other hand, the specificity of Zn−FL− Au NCs toward Cys was verified in presence of several interfering biomolecules (histidine, glycine, glutamic acid, aspartic acid, glucose, and lactose), cations (sodium ions), and anions (chloride ions). Further, to demonstrate the discrimination of Cys and Glu using Zn−FL−Au NCs in human blood serum, Cys and Glu were added to Zn−FL−Au NCs in blood serum and the corresponding effects on luminescence of Zn−FL−Au NCs were recorded. It was interesting to observe that the CIE coordinates of Zn−FL−Au NCs varied to a negligible extent upon interaction with these chemical species (Figure 3B). Interestingly, similar results as those obtained in buffer medium could be observed in serum as well (Figure S12A,B). However, a little variation was observed with regard to the intensity of the peak of Zn−FL−Au NCs at ∼510 nm, which in buffer medium got increased upon addition of Cys whereas in serum got decreased upon addition of Cys. Also, we could demonstrate that other amino acids, metal ions, and carbohydrates (control samples) had no discernible effect on 17224
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Scheme 1. Schematic Illustration of a Plausible Mechanism Responsible for Discrimination between Cys and Glu upon Interaction with Zn−FL−Au NCs
oxidized dimer of Cys, was added to Zn−FL−Au NCs and the corresponding effect on luminescence was recorded. The choice of cystine for the control experiment was made, as it serves both the purpose of lacking a free thiol group and having approximately twice the size of Cys. Interestingly, owing to the absence of a free thiol group, no discernible change in luminescence and chromaticity coordinates of Zn− FL−Au NCs could be observed upon interaction with cystine (Figure S15). This clearly highlights the pivotal role of thiol group in attachment to zinc ions in discrimination of Cys and Glu using Zn−FL−Au NCs. On the other hand, the larger size of cystine might have limited the ability of the same to displace Au NCs from Zn−FL−Au NCs, akin to Glu, which further caused no change in luminescence of Zn−FL−Au NCs. Thus, different modes of interaction of Zn−FL−Au NCs with Cys and Glu offered a facile route to chromaticity index-based discrimination of BTs at “a few particle” level. A plausible schematic illustration of the mechanism responsible for discrimination of BTs is depicted herein (Scheme 1).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Uday Narayan Pan: 0000-0002-8545-7223 Anumita Paul: 0000-0002-2999-9749 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge Department of Electronics and Information Technology, Government of India for financial assistance [No. 5(9)/2012-NANO (Vol. II)]. We are thankful to Central Instruments Facility, IIT Guwahati for providing instrumental facilities.
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CONCLUSIONS In a nutshell, we report an unprecedented strategy for discrimination of BTs (Cys and Glu) at “a few particle” level using dual-emitting nanoprobe-based changes in chromaticity coordinates of the sensor. The dual emitting nanoprobe was fabricated by zinc-mediated conjugation of red-emitting Au NCs and green-emitting FL. The luminescence of Au NCs in the composite was selectively quenched upon interaction with Cys, thereby altering the chromaticity coordinates of Zn−FL− Au NCs. On the other hand, the luminescence of Au NCs and FL was enhanced, albeit to a small extent upon interaction with Glu. Thus, the chromaticity coordinates of Zn−FL−Au NCs were affected to a lesser degree upon interaction with Glu as compared with interaction with Cys. Further, the discrimination between the aforementioned BTs has been achieved on the basis of differential chemical interaction of Zn−FL−Au NCs with Cys and Glu, which is envisioned to pave the ground for the advent of facile routes for distinction between chemically allied compounds.
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Characterization of gold nanoclusters, effect of addition of zinc acetate on luminescence of gold nanoclusters, characterization of Zn−FL−Au NCs, effect of addition of zinc acetate on luminescence of fluorescein, results of control experiments, results of concentration-dependent discrimination of biothiols (PDF)
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02373. 17225
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DOI: 10.1021/acsomega.8b02373 ACS Omega 2018, 3, 17220−17226
