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Turn-On and Near-Infrared Fluorescent Sensing for 2,4,6Trinitrotoluene Based on Hybrid (Gold Nanorod)(Quantum Dots) Assembly Yunsheng Xia,* Lei Song, and Changqing Zhu* Anhui Key Laboratory of Chemo-Biosensing, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China
bS Supporting Information ABSTRACT: In this study, we design a FRET system consisting of gold nanorod (AuNR) and quantum dots (QDs) for turn-on fluorescent sensing of 2,4,6-trinitrotoluene (TNT) in near-infrared region. The amine-terminated AuNR and carboxyl-terminated QDs first form a compact hybrid assembly through amine-carboxyl attractive interaction, which leads to a high-efficiency (>92%) FRET from QDs to AuNRs and an almost complete emission quenching. Next, added TNT molecules break the preformed assembly because they can replace the QDs around AuNRs, based on the specific reaction of forming Meisenheimer complexes between TNT and primary amines. Thus, the FRET is switched off, and a more than 10 times fluorescent enhancement is obtained. The fluorescence turn-on is immediate, and the limit of detection for TNT is as low as 0.1 nM. Importantly, TNT can be well distinguished from its analogues due to their electron deficiency difference. The developed method is successfully applied to TNT sensing in real environmental samples.
2,4,6-T
rinitrotoluene (TNT) is a highly explosive and environmentally detrimental substance that has been of pressing social concern. Therefore, improvements in methods for its sensing have attracted considerable research efforts in recent years.1 It is well-known that fluorescent technique bears the merits of high signal output and simplicity. Especially combination of near-infrared (NIR, 650-900 nm) and turn-on fluorescence can greatly improve the sensitivity due to the efficient decrease of background signals both from short wavelength (400-600 nm) emission in samples and from probe signals themselves.2 Unfortunately, the present designs for TNT sensing are almost focused on fluorescence quenching based on the electron-deficiency of TNT molecule and employ short wavelength emission signal.3 The resulted high background would limit the further sensitivity improvement. Meanwhile, similar quenching effects from coexisting TNT analogues, such as 2,4dinitrotoluene (DNT) and nitrobenzene (NB), often lead to poor selectivity. So, it is urgent to explore and create new methods for highly sensitive and selective TNT sensing. As a potential alternative, the NIR fluorescence resonance energy transfer (FRET) system based on quantum dots (QDs) and gold nanorods (AuNRs), which act as energy donor and acceptor (quencher), respectively, can provide a possibility of high-efficiency energy transfer due to high spectral overlap4,5 and unique energy transfer mode of multiple donors and single acceptor.6,7 In fact, the study of fluorescent quenching and sensing applications of AuNRs-QDs pair has been reported.8 However, whether it is possible to achieve turn-on fluorescent sensing using this system has remained an open question, because it suffers from r 2011 American Chemical Society
the great challenge in the design of surface chemistry of the particles. For turn-on sensing, the particle surface should meet two demands simultaneously. First, rational choice of modified reagents is needed to decrease the distance between donors and acceptor enough for high FRET efficiency. Next, the resulted FRET process must be switched off and give a specific response to analytes. To overcome the above problems, we modify AuNRs and QDs with short thiol-molecules, cysteamine and 3-mercaptopropionic acid (MPA), respectively. It is known that AuNRs synthesized by colloidal method are capped with a bilayer of cetyltrimethylammonium bromide (CTAB) molecules,5 and that layerby-layer technique8a and biomacromolecules8b are often employed in the modification of AuNRs. The resultant spacer with several nanometers thickness around AuNR surface has an adverse effect on the energy transfer, because the efficiency of FRET is inversely proportional to the sixth-power of the distance between donor and acceptor.6 The choice of cysteamine for the modification of AuNR surface is crucial, because it not only brings a compact surface structure but provides bind sites for QDs and TNT molecules. On the other hand, highly emitting QDs were directly synthesized using MPA as stabilizers.9 Our design is different from previous reports and is shown in Scheme 1: the amine-terminated AuNR (λabsorption = 666 nm) and carboxyl-terminated QDs (λemission = 667 nm) first form a Received: November 3, 2010 Accepted: December 31, 2010 Published: January 24, 2011 1401
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Analytical Chemistry Scheme 1. Structure of the Hybrid AuNR-QDs Assembly and Schematic Illustration of Its FRET-Based Operating Principle
tight assembly through amine-carboxyl attractive interaction, which leads to a high-efficiency (exceed 92%) FRET from QDs to AuNRs and an almost complete emission quenching. Next, added TNT molecules break the preformed assembly because they can replace QDs around AuNRs, based on the specific reaction of forming Meisenheimer complexes between TNT and primary amines.10 Thus, the FRET is switched off, and a more than 10 times fluorescent enhancement is obtained. The limit of detection for TNT is 0.1 nM. As far as we know, the detection limit achieved with our method is more than 400% lower as compared to the most sensitive fluorescent quenching method reported before.3m Importantly, TNT can be well distinguished from its analogues due to their electron deficiency difference. To the best of our knowledge, turn-on11 and NIR fluorescent sensing for TNT sensing have not been reported yet.
’ EXPERIMENTAL SECTION Materials. MPA (99%), cysteamine, and Te powder (-60 mesh, 99.999%) were purchased from Alfa Aesar. TNT, DNT, and NB were obtained from Sigma-Aldrich. HAuCl4 3 3H2O, CTAB, ascorbic acid, NaBH4, CdCl2 3 2.5H2O, AgNO3, HCl, NaOH, and other routine chemicals were acquired from Shanghai Reagent Co. All solutions were prepared with double deionized water. Caution: The highly explosive TNT should be used with extreme caution and handled only in small quantities. Instruments and Characterizations. A Hitachi-U-3010 spectrometer was used to record the UV-visible spectra. Fourier transform infrared (FT-IR) spectra were recorded from a KBr window on a Perkin-Elmer PE-983 FT-IR spectrophotometer. Steady-state fluorescence measurements were performed using a Hitachi F-4500 spectrofluorometer equipped with a R3896 red-sensitive multiplier and a 1 cm quartz cuvette. Fluorescence lifetime measurements were performed with the time correlated single photo counting technique on the combined steady state and lifetime spectrometer (Edinburgh Analytical Instruments, FLS920). Characterizations of transmission electron microscopy (TEM) were carried out on Tecnai G2 20 ST (FEI) under the accelerating voltage of 200 kV. The samples for TEM measurements were prepared by the deposition of one drop of aqueous dispersion on a copper grid coated with thin films of carbon, and the solvent was removed by evaporation in air. The solutions were analyzed for particle sizes and ξ-potential values using dynamic light scattering (DLS, Zetasizer Nano ZS
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series, Malvern Instruments) with 633 nm laser wavelength and a measurement angle of 173° (backscatter detection) at 25 °C. Supersonic instrument (Kunshan Hechuang KH-100B) was used for cysteamine-modified AuNRs. The structures of cysteamine and MPA molecules were characterized by GaussView software. Synthesis and Purification of CdTe/CdS Core/Shell QDs. MPA-capped CdTe QDs were prepared in aqueous solution using the method described previously.9 Briefly, 1.25 10-3 mol of CdCl2 3 2.5H2O was dissolved in 100 mL of water, and 3.0 10-3 mol of MPA was added under stirring, followed by adjusting the pH to 11.0 by adding dropwise 1.0 M NaOH. The solution was deaerated by N2 bubbling for 40 min. Under vigorous stirring, 1.6 10-4 mol of freshly prepared oxygen-free NaHTe was injected to the above solution. Afterward, the solution mixture was heated to 100 °C and refluxed for 8 h. We then used previous reports12 for CdTe/CdS core/shell QDs. A 50 mL crude CdTe solution was cooled to room temperature and deaerated by N2 bubbling for 30 min. Under vigorous stirring, 8.0 10-5 mol of thioacetamide (dissolved in 1 mL of water), which acted as sulfur source, was added to the QD solution. Next, the mixture solution was heated and maintained 80 °C in an oil bath for 6 h to form the CdTe/CdS core/shell structure. For purification of the core/ shell CdTe/CdS QDs, acetone was added dropwise until the 15 mL QD solution became turbid. The turbid dispersion was then left stirring for 15 min, and the precipitate containing CdTe/CdS QDs was isolated from the supernatant by centrifugation (6000 r/min), which was dried overnight under vacuum at 40 °C. Finally, the dried CdTe/CdS QDs were redispersed in 15 mL of water for stock solution, which was stable at least 8 months at 4 °C. The QD stock solution was further diluted to 1.0 10-7 M by water for fluorescent titration experiments. The concentration of CdTe/CdS QDs was estimated according to the concentration of CdTe core using Peng’s method.13 Synthesis and Modification of AuNRs. AuNRs were prepared in aqueous solutions using a seeded growth protocol.5a Specifically, the seed solution was first made by the addition of 0.01 M HAuCl4 solution (0.25 mL) into 0.1 M CTAB (9.75 mL) in a 15 mL plastic tube. The solution was gently mixed by inversion. A freshly prepared, ice-cold 0.01 M NaBH4 solution (0.6 mL) was then injected quickly into the mixture solution, followed by rapid inversion for 2 min. The resultant seed solution was kept at 30 °C for 2 h before use. To grow AuNRs, 0.01 M HAuCl4 (2 mL) and 0.01 M AgNO3 were first mixed with 0.1 M CTAB (40 mL) in a 50 mL plastic tube. The length-diameter ratio of AuNRs was controlled by the amounts of added Agþ. To obtain AuNRs-600, AuNRs-666, and AuNRs-798, 0.200, 0.288, and 0.440 mL of AgNO3 was added, respectively. 1.0 M HCl (0.8 mL) was then added for adjusting the pH of the growth solution, followed by the addition of 0.1 M ascorbic acid (0.32 mL). After the growth solution was mixed by inversion, the CTAB-stabilized seed solution (0.096 mL) was rapidly injected. The resultant solution was gently mixed for 10 s and left undisturbed 12 h in 30 °C water-bath. Amine-terminated AuNRs were obtained by a modified literature procedure.14 Briefly, 5 mL of each AuNR sample was centrifuged twice at 10 000 r/min for 15 min to remove CTAB and other ions in the solution. The precipitate was redispersed in water (5 mL), and subsequently an aqueous solution of cysteamine (20 mM, 0.5 mL) was added dropwise under vigorous stirring. The mixture was sonicated for 0.5 h at 50 °C, which was further heated in a 50 °C water bath for 3 h for the modification of both tips and sides of AuNRs.14 The resultant AuNRs were then collected by centrifugation twice at 7000 r/min 1402
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Analytical Chemistry for 15 min to remove excess cysteamine and desorbed CTAB. The purified amine-terminated AuNRs were dried overnight under a vacuum at 40 °C for FT-IR measurements. For fluorescent titration experiments, the purified cysteamine-modified AuNRs were dispersed in water directly without dry, whose concentrations were estimated by Lambert-Beer Law. The extinction coefficients of AuNRs with different aspect ratios were obtained on the basis of the report of Orendorff and Murphy.15 Evolutions of Fluorescence of QDs with Increasing AuNR Concentrations. Twenty microliters of purified CdTe/CdS core/shell QD solution (1.0 10-7 M) and various amounts of cysteamine-modified AuNRs were placed in a series of 5 mL colorimetric tubes. The mixture was diluted to 2 mL with water and mixed thoroughly. Ten minutes later, the mixed solutions were transferred separately into a 1 cm quartz cuvette. Their fluorescence spectra were recorded by operating the fluorescence spectrophotometer at an excitation wavelength of 380 nm. Procedures for TNT Sensing. Twenty microliters of purified CdTe/CdS core/shell QD solution (1.0 10-7 M) and 50 μL AuNRs (7.2 10-10 M) were placed in a series of 5 mL colorimetric tubes. Ten minutes later, various amounts of analytes were added. The mixtures were diluted to 2 mL with water and mixed thoroughly. Ten minutes later, the mixed solutions were transferred separately into a 1 cm quartz cuvette. Their fluorescence spectra were recorded by operating the fluorescence spectrophotometer at an excitation wavelength of 380 nm. Procedures for TNT Sensing in Real Samples. Tap water (from Lab) and lake water (Jinghu Lake, Wuhu) samples were used after filtration twice through 0.22 μM filters. For sensing of TNT in the water samples, pure and TNT-spiked water samples (30 μL) were added to an aqueous dispersion of (AuNR-QDs) assembly; the concentrations of AuNRs and QDs were 18 pM and 1.0 nM, respectively. Ten minutes later, the mixed solutions were transferred separately into a 1 cm quartz cuvette. Their fluorescence spectra were recorded by operating the fluorescence spectrophotometer at an excitation wavelength of 380 nm.
’ RESULTS AND DISCUSSION AuNRs typically exhibit two surface plasma resonance (SPR) modes. One is the transverse mode, and the other is the longitudinal mode. They correspond to electron oscillations perpendicular and parallel to the length axis, respectively. The longitudinal SPR band is very sensitive to the morphology of AuNRs and can be tuned from 600 to 1300 nm with their aspect ratio.5 In contrast, the SPR absorption peak of spherical gold nanoparticles (SAuNPs) is at about 520 nm and almost independent of their size.16 So, AuNRs instead of SAuNPs were chosen here, although most attention has been paid to the SAuNPs-QDs system.7,17 AuNRs with three different aspect ratios were synthesized by tuning the amounts of added Agþ,5a which were further functionalized with cysteamine.14 Because of the longitudinal SPR peak values, they are referred to as AuNRs-600, AuNRs-666, and AuNRs-798, respectively (Figure 1). The resultant amine-terminated AuNRs (Figures S1-S3 in the Supporting Information) not only act as scaffolds for negatively charged QDs but provide binding sites for TNT. As a kind of electron acceptor, TNT can interact with electron donors, typically primary amines, through the electron transfer mechanism.10 Such an interaction has been recently employed for designing the surface chemistry of NPs to achieve TNT sensing based on colorimetry,1f surface enhanced Raman scattering,1j and fluorescent quenching.3p,3s CdTe/CdS
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Figure 1. Normalized SPR absorption spectra of AuNRs with three different aspect ratios and emission spectrum of the CdTe/CdS core/ shell QDs.
Figure 2. TEM images of the hybrid AuNR-QDs assembly before (A) and after (B) addition of 66 nM TNT. The concentrations of AuNRs666 and QDs are 18 pM and 1.0 nM, respectively. QDs are marked by circles of dotted line.
core/shell QDs with an emission peak at 667 nm (Figure 1) were synthesized on the basis of a previous method.9,12 MPA was chosen as stabilizer because of its negative charge and short length (Figures S5D and S7 in the Supporting Information). It is known highly emissive NIR CdTe QDs can be directly synthesized using short thiol-molecules as stabilizers in aqueous solution.9 However, Te atoms on the surface of QDs are sensitive to oxygen and some heavy metal ions, leading to the instability of their emission.9a,12a,18 To overcome this problem, preformed CdTe QDs were coated with a CdS layer. As demonstrated by TEM shown in Figure 2A, positively charged AuNR can interact with negatively charged QDs and form a hybrid assembly via electrostatic interaction.17a Analysis suggests that there are no less than 20 QDs at the sides of one AuNR. However, all the QDs cannot be found if they just locate in the projection of AuNR because of the big differences in their contrast and size. So, it is reasonable to estimate that each AuNR is surrounded by more than 40 QDs. On the other hand, the system contains 18 pM of AuNRs and 1 nM of QDs, so the ratio of AuNRs to QDs is 1:55, which generally matchs the TEM data. As expected, the emission of QDs is quenched by AuNRs-666 because of the FRET from QDs to AuNRs (Figure 3). The FRET efficiencies are estimated by the value E = 1 - FDA/FD,6 where FDA is the integrated fluorescence intensity of the donor in the presence of the acceptor and FD is the integrated fluorescence intensity of the donor alone (no acceptors present). As shown in Figure 4, the FRET efficiency can exceed 92% as the concentration of AuNRs-666 reaches 18 pM. Such high FRET efficiency is 1403
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Analytical Chemistry
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Figure 3. Evolutions of fluorescence spectra of CdTe/CdS core/shell QDs with increasing AuNRs-666 concentrations containing 1.0 nM QDs. The AuNRs-666 concentrations from top to bottom are 0, 1.8, 3.6, 5.4, 7.2, 9.0, 10.8, 12.6, 14.4, 16.2, and 18.0 pM, respectively.
Figure 5. Evolution of emission spectra of AuNR-QDs assemblies with increasing TNT, whose concentrations from bottom to top are 0, 1.1, 5.5, 11, 22, 33, 44, 55, and 66 nM, respectively. The concentrations of AuNRs-666 and QDs are 18 pM and 1.0 nM, respectively. The dashed line represents the emission of the QDs alone (no AuNRs and TNT).
Figure 4. Comparison of FRET efficiencies of AuRs-600, AuNRs-666, and AuNRs-798. The concentration of QDs is 1.0 nM.
attributed to a combination of factors, as considering the characteristics of the present design and the rule of FRET efficiency (eqs 2 and 3 in the Supporting Information). First, AuNRs have a large extinction coefficient. On the basis of previous reports,15,19 the extinction coefficient of AuNRs is more than 109 M-1 cm-1, which is about 4-5 orders of magnitude higher than that of organic dyes. Second, AuNRs have larger surface area and lower curvature, which can increase the quenching sites and improve the quenching efficiency.6,7,8c Just as described above, the hybrid assembly is composed of single AuNR and more than 40 QDs, as such (multiple donors)-(single acceptor) motif facilitates FRET efficiency.7,8c In contrast, there are only 5 QDs around each SAuNP as they form the assembly.7 Third, the spectra of the AuNRs and QDs overlap completely (Figure 1), which benefits from the large tunability of the spectrum bands of the two particles. Obviously, the greater is the spectrum overlap between donor and acceptor, the greater is the energy transfer efficiency in FRET system. Last, the most impotant reason is the close proximity of AuNRs to QDs, because FRET efficiency is inversely proportional to the sixth-power of the distance between donor and acceptor. In the present system, short thiol-molecules (cysteamine and MPA) are chosen for the modification of NPs’ surface, as the sum of their length is smaller than 1 nm (Figure S7 in the Supporting Information). Recently, Zhu and co-workers observed only 13% energy transfer efficiency as the FRET from QDs to AuNRs was modulated by antigen-antibody interaction.8b The lower FRET efficiency might arise from the relatively far distance between energy donor and acceptor because of the large size of the biomacromolecules. Figure 4 shows the QDs can also be quenched by
AuNRs-600 or AuNRs-798, but the FRET efficiencies are obviously lower than that of AuNRs-666, indicating that spectrum overlap is another important role in the mediation of the FRET. Because of the highest FRET efficiency, AuNRs-666 is employed in the following studies. We then introduced TNT to the solution containing AuNRQDs assemblies. As shown in Figure 5, the quenched emission shows a gradual recovery with the increase of added TNT. A 10.6 times fluorescent enhancement is observed as the concentration of TNT reaches 66 nM. The reason for fluorescent enhancement is that TNT molecules break AuNR-QDs assemblies and switch off the FRET from QDs to AuNR (Scheme 1 and Figure 2B). The interaction of TNT molecules with AuNR-QDs assemblies was then studied in situ by DLS technique. As shown in Figure S9 (Supporting Information), the size of AuNR-QDs assemblies decreases gradually with the increase of introduced TNT molecules, which is in agreement with the TEM result (Figure 2). As mentioned above, TNT molecules can react with primary amines and form Meisenheimer complexes (Figure S10 in the Supporting Information), and such a covalent interaction20 is stronger than electrostatic binding between amine and carboxyl groups, so QDs around the AuNRs are replaced by TNT molecules. As shown in Figures 3 and 5, the fluorescence maximum peak shifts from 667 to 672 nm in the quenching process and then returns almost completely (from 672 to 669 nm) in the recovery process. The reason for the emission shift may be due to that the smaller QDs are quenched more effectively than larger ones, because the smaller QDs have larger quenching radius.21 The quenching and recovery processes were also studied by timeresolved fluorescence spectroscopy (Figure S11 in the Supporting Information). A decrease of decay time is observed as AuNRs are added to QD solution, which originates from the additional decay channels provided by AuNRs (energy acceptor).6 Next, a good degree of life recovery is observed when TNT is introduced, suggesting that the additional decay channels abate with the disintegration of the hybrid assemblies. This result is consistent with the observations of steady-state fluorescent spectra, TEM, and DLS. In the present design, the binding sites of TNT are rationally put on the surface of AuNRs, because TNT can quench QDs 1404
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Analytical Chemistry
Figure 6. (A) Fluorescence response of AuNR-QDs assemblies to various cations and nitro aromatic compounds. The concentrations of Hg2þ, Agþ, Cu2þ, NB, and DNT are 0.11 μM, other metal ions are 11 μM, TNT-1 is 11 nM, and TNT-2 is 55 nM. (B) The selectivity of AuNR-QDs assemblies for TNT in the presence of cations and other nitro aromatic compounds. The concentrations of Hg2þ, Agþ, Cu2þ, NB, and DNT are 0.11 μM, while other metal ions are 11 μM. F0 and F are the fluorescence intensities of the AuNR-QDs assemblies before and after adding the corresponding substances, respectively. The concentrations of AuNRs-666 and QDs are 18 pM and 1.0 nM, respectively.
through electron transfer as they bind to each other. 3p,3s The dose-dependent fluorescent enhancement (Inset of Figure 5) exhibits a good linearity that can be expressed as I = 4.5CTNT þ 39.5 (r = 0.998) in the TNT concentrations range of 1.1-66 nM. The fluorescent signal recovery follows a simple linear dependence on the TNT concentrations in a wide range, which is different from a previous report3k and indicates TNT does not quench the CdTe/CdS QDs. This result is further demonstrated by the control experiment (Figure S12 in the Supporting Information). The detection limit, calculated by a 5% change of signal as a detectable standard,3p is 0.10 nM (0.02 ppb), which is lower more than 400% as compared to the most sensitive fluorescent quenching method reported before3m (Table S1 in the Supporting Information). It is known that TNT is a quencher for most fluorophores (organic dyes and QDs). Previous reports have demonstrated that the QDs can be quenched by relatively high concentration (μM) of TNT even without binding sites for TNT on QDs’ surface.3p,3t So, we further studied the effect of TNT in a broader concentration range on the emission of AuNR-QDs assemblies. As shown in Figure S13 (Supporting Information), higher concentrations of TNT molecules can quench the recovered QDs because of the collision effect between TNT molecules and QDs. For example, the emission of recovered QDs is quenched about 50% after 4400 nM of TNT is added, which is in agreement with
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the previous observations. However, the QDs cannot be quenched if the concentration of TNT is less than 70 nM (Figure S12 in the Supporting Information), even in the absence of cysteaminemodified AuNRs. These results indicate that the present turn-on mode using AuNR-QDs assemblies as probes is very competent for ultra trace sensing of TNT. To assess the selectivity of the present method for TNT, the effects of common interfering agents, such as NB and DNT, on TNT sensing were first examined (Figure 6 and Figure S15 in the Supporting Information). The results show that there is no response when concentrations of NB and DNT molecules change from 1 to 110 nM, and only a little quenching effect as their concentrations further increase, thus indicating that these analogues do not interfere with the TNT sensing. The reason of the excellent selectivity of our method over NB and DNT may be due to that their electron deficiencies are not high enough for competing with carboxyl-terminated QDs around AuNRs, because there are only one and two electron-withdrawing nitro groups in each aromatic ring, respectively.1j For real applications, especially in environmental samples, there can be impurities due to the metal ions. As a result, we then tested the selectivity of our probes in the presence of alkali, alkaline earth (Naþ, Kþ, Mg2þ, Ca2þ, Ba2þ, Al3þ), and transition heavy metal ions (Pb2þ, Fe3þ, Co2þ, Zn2þ, Cd2þ, Agþ, Cu2þ, Hg2þ). As shown in Figure 6, most metal ions at a concentration of 11 μM have no measurable effect on 11 nM TNT sensing using the AuNR-QDs as probes. 1000 times higher coexisted metal ions do not interfere with the sensing of TNT. It has already been documented that Hg2þ, Cu2þ, and Agþ can quench QDs heavily.18 It is found that coating CdTe QDs with a CdS layer decreases the quenching effects of these ions. As shown in Figure 6, when the concentrations of Cu2þ, Hg2þ, and Agþ are 10 times higher than that of TNT in solution (110 nM vs 11 nM), they show a negligible effect on detection of TNT. The interference from higher concentrations of these metal ions can be eliminated effectively by removing them using mercapto-cotton.22 The effect of ionic strength should also be considered in the present system because the formation of AuNR-QDs assemblies is mainly based on electrostatic interaction. As shown in Figure S16 (Supporting Information), the fluorescence of the assemblies in the absence of TNT has only a little enhancement ( 1 mM because of the partial precipitation of the AuNR-QDs assemblies in a higher concentration of electrolyte (Figure S16 in the Supporting Information).23 So, NaCl does not affect TNT sensing as its concentration is
