Fluorescent Gold Nanocluster-Based Sensor Array for Nitrophenol

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Fluorescent Gold Nanocluster-based Sensor Array for Nitrophenol Isomer Discrimination via an Integration of Host-Guest Interaction and Inner Filter Effect Hongwei Yang, Fengniu Lu, Ye Sun, Zhiqin Yuan, and Chao Lu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b03394 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 13, 2018

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Analytical Chemistry

Fluorescent Gold Nanocluster-based Sensor Array for Nitrophenol Isomer Discrimination via an Integration of Host-Guest Interaction and Inner Filter Effect Hongwei Yang,† Fengniu Lu,‡ Ye Sun,† Zhiqin Yuan,*,† and Chao Lu*,† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China ‡

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan Email: [email protected] (Z. Yuan); [email protected] (C. Lu). ABSTRACT: The rapid discrimination of nitrophenol isomers has been a long-standing challenge because of the tiny structural differences among the isomers. In this study, a fluorescent sensor array based on three different-color emitting gold nanoclusters (Au NCs) that were functionalized with three different ligands and a co-capping ligand β-cyclodextrin (β-CD) has been constructed for the facile discrimination of three nitrophenol isomers via the linear discriminant analysis of isomer-induced fluorescence quenching patterns. The fluorescence quenching occurs in two steps: firstly, β-CDs adsorb nitrophenol isomers onto the surface of Au NCs via host-guest interaction; secondly, each nitrophenol isomer quenches the fluorescence of a specific type of Au NCs through an inner filter effect. The different binding affinities between β-CD and each nitrophenol isomer, as well as the distinct quenching efficiencies of the isomers on the fluorescence of each Au NCs, enable an excellent discrimination of the three isomers at a concentration of 5 µM, when linear discriminant and hierarchical cluster analyses were smartly combined. In addition, even a mixture of two isomers could be distinguished with the proposed sensor array. The practicability of this developed sensor array is validated by a high accuracy (98.0%) examination of 51 unknown samples containing a single isomer or a mixture of two isomers.

N

itrophenols are typical aromatic nitro compounds that have won wide applications in different areas including pharmaceuticals, pesticide industries, and the production of fine chemicals.1-2 However, the highly stable residues of nitrophenols remained in the environment are harmful to human health because they can cause severe diseases such as eye irritation and cyanosis.3-4 It has been well known that the three isomers of nitrophenols, namely, o-nitrophenol (ONP), mnitrophenol (MNP) and p-nitrophenol (PNP), exhibit different toxicities.5 Therefore, the discrimination and selective detection of each nitrophenol isomer is essential to environmental control and human health protection. To date, a series of fluorescence and/or absorption spectroscopic methods has been reported for the detection of nitrophenols based on molecular imprinted polymer technique or host-guest chemistry.612 These fluorescence-based methods with high sensitivities generally rely on the quenching capability of nitrophenols via the inner filter effect, i.e., a spectral overlap between the absorption of nitrophenols and the excitation and/or emission of the fluorophores.6,9 However, most of these fluorescent sensors show a poor discrimination among the three isomers. Therefore, it is of great significance to develop a facile method for the specific recognition of each nitrophenol isomer. Array-based sensing methods, also called “chemical nose/tongue” strategies, have been widely applied in the dif-

ferentiation of analytes of similar structures.13 A sensor array generally consists of several sensing elements, whose interactions with different analytes can generate different response patterns that are distinguishable by linear discriminant analysis (LDA). This principle allows the identification of analytes with small structural/chemical differences through a smart cooperation of versatile sensing systems.14-18 Due to their easy preparation and reactive properties, gold nanoclusters (Au NCs) are widely used for constructing array-based sensing systems.19-29 For instance, Au NCs-based sensing arrays can easily identify cells and proteins.30-32 In general, the sensing of Au NC-based arrays utilizes the selective rather than specific interactions between the receptors and analytes. To further enhance the differentiation capability of sensor array, the specific recognition between a receptor and an analyte should be used, which is theoretically feasible.33 As a proof-of-concept, we herein constructed a novel Au NC-based fluorescent sensor array for efficient discrimination of three nitrophenol isomers. Three different types of Au NCs with diverse excitation and emission profiles were synthesized by introducing different capping ligands (histidine, glutathione, and 11-mercaptoundecanoic acid) onto the surface of Au NCs, each of which exhibits different inner filter effects toward three nitrophenol isomers. In addition, all the Au NCs were cofunctionalized with β-cyclodextrin (β-CD) to take full ad-

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vantage of the host-guest interactions between β-CD and nitrophenols. The sensing principle of the dual-ligands functionalized Au NCs sensor array is based on a concerted integration of the different binding affinities between the surface β-CDs and the nitrophenol isomers and the different fluorescence quenching capabilities of three isomers toward each Au NCs. The strategy exhibited a substantial promise for the discrimination of individual isomer and also the mixture of two isomers. To the best of our knowledge, this is the first demonstration of an artful integration of host-guest interaction and inner filter effect for nitrophenol isomer discrimination. The practical application of the prosed Au NCs sensor array was validated by high accuracy (98.0%) examination of 51 unknown samples of single isomer or a mixture of two isomers.

EXPERIMENTAL SECTION Chemicals. Tetrakis (hydroxymethyl) phosphonium chloride (THPC), 11-mercaptoundecanoic acid (MUA), mercaptopropionic acid (MPA), L-glutathione reduced (GSH) and 4Nitro-o-phenylenediamine (4-NOPDA) were purchased from Sigma-Aldrich (Milwaukee, USA). Chloroauric acid tetrahydrate (HAuCl4·4H2O) was purchased from damas-beta (Shanghai, China). L-histidine (His) was purchased from Solarbio (Shanghai, China). Anhydrous ethanol and sodium hydroxide (NaOH) were obtained from Beijing chemical (Beijing, China). Mono-(6-Mercapto-6-deoxy)-beta-cyclodextrin (β-CD-SH), β-cyclodextrin (β-CD) and phenol (PhOH) were purchased from HWRK CHEM (Beijing, China). oNitrobenzoic acid (o-NBA) was purchased from TCI (Shanghai, China). Three nitrophenol isomers, o-nitrophenol (ONP), m-nitrophenol (MNP), p-nitrophenol (PNP) were purchased from Innochem (Beijing, China). 2,4,6-Trinitrotoluene (TNT) solution was purchased from AccuStandard (New Haven, USA), o-Dihydroxybenzene (o-DOHB) and hydroquinone (HQ) were purchased from Fuchen Chemical (Tianjing, China). All chemicals were used without further purification. Ultrapure water (18.2 MΩ) was obtained from a Millipore system. All glassware was cleaned by fresh aqua regia. Phosphate buffer (PB, pH 7.4, 10 mM) and carbonate buffer (pH 9.2, 0.2 M) were prepared according to a standard handbook. Synthesis of β-CD Involved Dual-Ligands Functionalized Au NCs.

Three dual-ligands functionalized Au NCs were prepared through ligand exchange approach, as illustrated in Scheme 1. Au NC1 capped with β-CD and MUA was synthesized using our previously reported method with slight modification.23 Briefly, 200 µL NaOH (1 M) solution and 48 µL THPC (8%, wt) solution were first mixed with 16 mL ultrapure water under violent stirring for 5 minutes, and then 800 µL HAuCl4 (Au3+, 25 mM) solution was added rapidly. One minute later, 200 µL β-CD-SH (10 mM) solutions were added to obtain βCD protected gold nanoparticles (β-CD-Au NPs). After stirring for another 15 minutes at room temperature, the solution was cooled to 4 °C for overnight. After aging, 1 mL of β-CDAu NPs stock solution was mixed with 100 μL of carbonate buffer (pH 9.2, 0.2 M) and 75 μL of MUA solution (0.1 M, in ethanol) in a thermomixer for 2 hours to obtain Au NC1. The produced Au NC1 was purified through ultrafiltration use a 10 kDa cut-off ultrafiltration tube to remove excess reagents. For preparing Au NC2 an Au NC3, two Au NCs precursors capped with His and GSH, denoted as His-Au NCs and GSH-Au NCs were synthesized according to previous reports.34-35 The generated His-Au NCs and GSH-Au NCs solution were treated with β-CD-SH (5 mM, final concentration) under stirring at 50 °C for 3 hours. After then, the resulted solutions were filtered with 3 KDa (Au NC2) and 10 kDa (Au NC3) cut-off ultrafiltration tubes to remove any unwanted reactants or byproducts. All three Au NCs were stored at 4 °C for subsequently use Characterization. UV−vis absorption spectra were recorded on a UV-2401PC (Shimadzu, Japan). Fluorescence spectra were recorded using a F-7000 fluorescence spectrophotometer (Hitachi, Japan). Fourier transform-infrared spectra (FT-IR) were obtained by using a Nicolet 6700 infrared spectrophotometer (Thermo, USA). Hydrodynamic diameter and zeta potential measurements were performed with a Zetasizer Nano-ZS (Malvern, U.K.). The X-ray photoelectron spectroscopic (XPS) data were collected on a ESCALAB 250 X-ray photoelectron spectrometer with an Al Kα = 280.00 eV excitation source (Thermo, USA). Transmission electron microscopy (TEM) was performed on a HT7700 transmission electron microscope (HITACHI, Japan). The time-resolved fluorescence decay curve was performed on FLS 980 (Edinburgh,

Scheme 1. Schematic illustration of synthesis of β-CD involved dual-ligands functionalized Au NCs.


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Analytical Chemistry U.K.). The Au NCs solution was dried using a FD5-series freezedryer (SIM, USA). Array-based Sensing of Nitrophenol Isomers. In the nitrophenol isomer sensing study, three isomers stock solution (1 mM, PB) were first prepared. For isomer (final concentration of 50 µM) discrimination, each Au NC solution (from Au NC1 to Au NC3 were 25, 50 and 125 µL, respectively) was mixed with 50 µL isomer solution and certain PB to make a 1 mL mixture, and fluorescence spectrum was recorded. The relative fluorescence variation ((I-I0)/I0) was used as the fluorescence response, where I0 and I are the fluorescence intensities of Au NC solution in the absence and presence of the isomers. This process was repeated for three isomers to generate five replicates of each. And the three isomers were tested against the three Au NCs array five times, to provide a 3 × 3 × 5 training data matrix. Since the emission profiles of three Au NCs were different, we thus chose the fluorescence intensity of corresponding maximum emission wavelengths as raw data during all LDA treatments to minimize the data analysis error. While, for 10 µM isomer differentiation, each Au NC solution was mixed with 10 µL isomer solution and certain 1× PB to make a 1 mL mixture. Data analysis process is the same as that mentioned above. To test the unknown isomers, stock solutions of three isomers were firstly prepared. Then, Au NC solution was added into a well and certain of randomly selected isomer solution were added to make the final concentration of 10 μM. Two isomers mixtures were tested using similar approach by fixing the total concentration of 10 μM. The fluorescence spectrum was recorded, and the relative integrated fluorescence intensity variation ((I-I0)/I0) of the maximum emission was used to generate the fluorescence response pattern. The resulted fluorescence response was analyzed with LDA to identify the tested isomer or isomer mixture by comparing the results with corresponding training matrix. We took the river water of Yuan Dadu as the real sample. After simple pretreatment (high-speed centrifugation (10000 rpm/5 min) to remove a small amount of sediment, 0.22 μm filter filtration and pH adjustment), the same analysis protocol was conducted to distinguish the three nitrophenol isomers. Similarly, 500 μL simply processed river water was added to Au NC buffer solution to make a 1 mL mixture, and the fluorescence response patterns of three isomers were then analyzed by LDA.

RESULTS AND DISCUSSION Synthesis and Characterization of β-CD Involved Dual-Ligands

Functionalized Au NCs with Diverse Excitation and Emission Profiles. As a starting point, we synthesized three β-CD involved dual-ligands functionalized Au NCs via ligand exchange strategy, as shown in Scheme 1. These three Au NCs were characterized by UV−vis spectrometry, steady-state and time-resolved fluorescence spectrometry, TEM, FT-IR spectrometry, dynamic light scattering analysis, Zeta potential measurements, XRD and XPS. The hydrodynamic diameters of those produced Au NCs are 2.6±0.1, 1.4±0.1, and 2.3±0.1 nm, respectively (Figure S-1, Supporting Information) As can be seen from the TEM images (Figure S-2, Supporting Information), these Au NCs had spherical shapes and the average diameters of the three Au NCs are 2.0±0.4, 0.9±0.3, and 1.8 ±0.3 nm, respectively. The hydrodynamic diameter of all Au NCs is larger than that of the TEM result due to the hydration of the surface ligands. 36 Also, according to the adsorption spectra of Au NCs solution, characteristic surface plasmon resonance absorption peak around 520 nm was not observed (Figure S-3, Supporting Information), revealing the absence of large Au NPs.37 Meanwhile, no characteristic gold crystal lattice peaks were displayed from the XRD patterns of three Au NCs,(Figure S-4, Supporting Information) further demonstrating the presence of small Au NCs only.38 As indicated in Figure 1, both excitation and emission intensities increased after β-CD-SH treatment, and the corresponding photographs of Au NCs solution also displayed brighter emission upon adding β-CD-SH, suggesting the successful surface modification of β-CD. It was noticed that the fluorescence excitation and emission profiles showed ignorable change, indicating the β-CD modification didn’t cause the structure variation of Au NCs.39 In other words, only surface ligand exchange happens. In addition, treatment of Au NCs precursors with conventional β-cyclodextrin (no thiol group) didn’t cause any fluorescence enhancement (Figure S-5, Supporting Information), suggesting surface ligand exchange by βCD-SH is responsible for the fluorescence enhancement. To further verify the successful surface functionalization with βCD, FT-IR spectra of Au NCs were performed (Figure S-6, Supporting Information). It can be seen that FT-IR spectra of all Au NCs possess a visible peak around 580 cm-1 assigned to the C-S stretching vibration. Since histidine has no thiol group, this should be attributed to the surface β-CD-SH, which means ligand exchange indeed happen. Also, characteristic peak at ∼1031 cm-1 belongs to coupled C–O/C–C stretching/O–H bending vibrations group of β-CD appeared in all three Au

Figure 1. Fluorescence spectra of MUA-Au NC (a), His-Au NC (b), and GSH-Au NC (c) without (green line) and with (red line) the addition of β-CD-SH. Inset images are corresponding photographs of three Au NC solutions in the absence (left) and presence (right) of βCD-SH under 365 nm UV light irradiation.



NCs, implying the surface coating with β-CD-SH.5 According the FT-IR spectra, the other ligand is still exists on the Au NC surface. As can be seen, an extra C–H stretch bond located at ∼2840 cm−1 that was assigned to –CH2– revealed the existence of MUA on Au NCs1 surface. Also, weak peak around ∼3200 cm-1 belongs to –NH2 (N–H stretch bond) suggested the existence of His on AuNC2 surface. In addition, a clear peak around ∼1660 cm-1 assigned to C=O reveals GSH remain on Au NC3 surface. Remain of the other ligand was also proved by XPS results.(Figure S-7 Supporting Information) A peak around 400 eV belongs to N(1s) was observed in both Au NC2 and Au NC3, revealing the His and GSH are still exist. Taken together, we indeed obtained three β-CD involved dualligands stabilized Au NCs. It is reported that high surface thiol density can induce the increase of fluorescence lifetime, and subsequently the enhancement of fluorescence intensity.34, 40 To find out the possible reason of β-CD-SH caused fluorescence enhancement, time-resolved fluorescence spectra of three Au NCs before and after β-CD modification were measured. As shown in Figure S8 (Supporting Information), β-CD functionalization leads to slight increase on fluorescence lifetime, suggesting β-CD functionalization can diminish the nonradiative transition. This result is accordance with the previous reports. In a word, these results indicate the successful preparation of three Au NCs with diverse excitation and emission profiles. The surface properties of Au NCs are important to analyte recognition, and diverse surface compositions show different binding affinities to various analytes. Since β-CD-SH was successfully connected to Au NC surface, analytes with small difference might be recognized via host-guest interaction. Therefore, we hypothesize that these characteristics should provide the produced Au NCs with selective binding capabilities and hence different interactions with small molecules, for example, nitrophenol. Nitrophenol Induced Fluorescence Quenching. The fluorescence profile change of the Au NCs in the presence of three nitrophenol isomers was first investigated. The fluorescence behavior of Au NCs showed dramatic changes in intensity but without variation on emission profile upon adding nitrophenol isomers to Au NC solution. As shown in Figure S-9 (Supporting Information), the addition of same concentration of different isomers to three Au NCs resulted in large variations of the fluorescence intensity, and continuous decrease of fluorescence intensities with the increasing nitrophenol concentration was displayed (Figure S-10, Supporting Information). This result indicated that the fluorescence behaviors of Au NCs could be affected by the interaction between nitrophenol isomer and Au NCs. As is known, ligand-analyte interaction can be largely affected by hydrogen bonding, electrostatic attraction and hydrophobic interaction, etc.41 To understand the interaction between Au NC and nitrophenol isomers, the surface charges of Au NCs were first tested by measuring the zeta potential. Because of the existence of abundant carboxyl group on the surface of Au NCs, negatively charged surface appears in PB (Figure S-11, Supporting Information). However, a part of nitrophenol isomers are also negatively charged at pH 7.4 due to the deprotonation of phenol group. In this case, the electro-

static repulsion between Au NC and nitrophenol isomer makes them difficult to get close. In addition, the electrostatic force between neutral nitrophenol isomers and Au NCs is negligible. Also, the hydrogen bonding capabilities between them are too weak to form stable complex. The binding constants of β-CD to ONP, MNP and PNP are reported to be 37.8, 44.2 and 90.1 M-1, respectively.43 Thereby, binding affinity between β-CD and nitrophenol isomers via host-guest chemistry plays important role on the interaction between Au NC and nitrophenol isomers.8, 42 Moreover, the fluorescence lifetimes of all three Au NCs showed negligible decrease upon the addition of nitrophenol isomers (Figure S-12, Supporting Information), ruling out the fluorescence resonance energy transfer or photoinduced electron transfer mechanism.36, 44 Furthermore, the absorption profiles of nitrophenol isomers displayed different overlap patterns to excitation spectra of three Au NCs. It is well known that nitrophenol can quench the fluorescence of inorganic emitters through inner filter effect.45-46 The different patterns suggest various inner filter effect efficiencies between isomer and Au NC exist (Figure S-13, Supporting Information). According to Figure S-13b and c (Supporting Information), absorption spectra of PNP showed maximum overlap to excitation spectra of Au NC2 and Au NC3, which is consistence with that of fluorescence quenching efficiency. Compared to Au NC2 and Au NC3, Au NC1 showed better overlap character with MNP (Figure S-13a, Supporting Information), which shows highest fluorescence quenching capability. It is noticed that the presence of high concentration nitrophenol isomers not only led to large fluorescence quenching but also variation of fluorescence excitation of Au NCs (Figure S-14, Supporting Information), suggesting inner filter effect indeed the reason cause fluorescence decease.47 Therefore, nitrophenol induced fluorescence quenching mechanism consists of two steps: first, β-CD adsorbs nitrophenol onto Au NC surface via host-guest interaction; second, nitrophenol quenches the fluorescence of Au NC through inner filter effect. Nitrophenol Isomer Discrimination. Since a certain nitrophenol causes the fluorescence quenching of Au NCs, and β-CDinvolved dual-ligands functionalization would affect the emission behaviors and generate apparent fluorescence profiles. Then, we tried to use these three Au NCs to construct the sensor array. One of the main advantages of array-based sensing is that it does not require differentiation of drivers. Ana-

Figure 2. Relative fluorescence increase (I-I₀)/I₀ of patterns of the Au NCs array against three nitrophenol isomers as an average of five parallel measurements.


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Analytical Chemistry lyte-receptor interactions are understood mechanically. As a proof of concept system, we created a sensor array consisting of three Au NCs for the identification of three nitrophenol isomers. To ensure that we were able to detect and distinguish isomer, we chose a 50 μM nitrophenol isomer to obtain the fluorescence response pattern. That is, for each nitrophenol isomer, we recorded its fluorescence response against the three Au NCs array for five repeats, producing a 3 × 3 × 5 matrix. (Figure S-15, Supporting Information). The dramatic difference on relative fluorescence increase patterns (I-I0)/I0 induced by three nitrophenol isomers suggests the feasibility of using this sensor array for three isomer recognition since the response pattern of each isomer generated by sensor array is generally unique. To further analyze the fluorescence response pattern of Au NC array against the three nitrophenol isomers, the obtained raw data were subjected to LDA using SPSS software version16.0. The variance ratios of event and data separation are related to the linear combination of features that differentiate serial classes of events or objects. The LDA can recognize the features and reduce the size of training matrix (3 Au NCs × 3 isomers × 5 replicates), and subsequently transform them into canonical factors. On the basis of these factors, the resulted canonical patterns were clustered into three different groups. It was seen from Figure S-16a (Supporting Information), the patterns were visualized as a well-clustered two-dimensional score plot with 100% of the total variance. And then, hierarchical cluster analysis (HCA), a statistical classification method based on Euclidean distance, was used to determine the similarity of nitrophenol isomers. All of the 15 cases (3 isomers × 5 replicates) were correctly assigned to their respective groups (Figure S-16b, Supporting Information). As can be seen, particular isomer induced fluorescence response patterns were characteristic and highly reproducible, indicating the excellent reproducibility of isomer identification. Compared to conventional array-based methods, this work utilizes specific acceptors for selective recognition of targets, offering an enhanced differentiation capability. To achieve sensitive isomer discrimination, a 10 μM nitrophenol isomer was also chosen to obtain the fluorescence response pattern. As shown in Figure 2, the distinct difference on relative fluorescence increase patterns (I-I0)/I0 induced by three nitrophenol isomers suggests the feasibility of using this sensor array for three isomer recognition even at 10 μM concentration. The

generated canonical patterns could be clustered into three different groups. And the score plot constructed using the first two canonical factors obtained from LDA analysis showed excellent clustering and satisfied dispersion with 100% of the total variance (Figure 3a). Meanwhile, the similarity of nitrophenol isomers was analyzed using HCA. As expected, all of the 15 cases (3 isomers × 5 replicates) were correctly assigned to their respective groups (Figure 3b). On the basis of principal component analysis, the exact contributions of three sensor elements were calculated to be 53.0% (Au NC1), 36.4% (Au NC2), and 10.6% (Au NC3), respectively. In order to verify the necessity of the surface ligand β-CD for the nitrophenol isomer discrimination, the identification capabilities of sensor array constructed from three Au NCs without surface ligand βCD or with surface ligand mercaptopropionic acid (MPA) were studied. The nitrophenol isomers (10 μM) could not be distinguished without surface ligand β-CD (Figure S-17a, Supporting Information). Also, surface modification with MPA showed poor discrimination capability to these nitrophenol isomers (Figure S-17b, Supporting Information). The sensing mechanism requires the collaboration of host-guest interaction and inner filter effect. As expected, these three Au NC-based sensor arrays could not distinguish the three nitrophenol isomers well. Thus, these results demonstrated that it is necessary for modifying β-CD onto to Au NC surface in order to distinguish the three nitrophenol isomers well. With the use of proposed sensor array, nitrophenol isomers can be discriminated even at 5 μM level (Figure S-18, Supporting Information). The sensing performance of our sensor array was also studied by determining the limit of detection (LOD) of nitrophenol isomers. We tried to quantify the three nitrophenol isomers by using total Euclidean distances (EDs = square root of the sum of the squared of the normalized (I₀-I)/I₀ values), which is based on a previous report.15 As shown in Figure 4, a good linearity range from 1 to 50 μM was observed using PNP as example. The limit of detection was calculated to be 0.21 μM based on 3σ rule. The LOD is below the minimum allowable amount in drinking water of 0.43 μM (given by the US Environmental Protection Agency).48 Similarly, the relationships between EDs and ONP or MNP concentration were also investigated (Figure S-19, Supporting Information). To validate the substantial promise for the discrimination of isomers, the mixtures of two isomers with different molar ratios (total isomer concentration was 10 μM) was subsequently

Figure 3. (a) Canonical score plots for the first two factors of relative fluorescence increase (I-I₀)/I₀ pattern from three isomers analyzed by LDA. (b) HCA analysis of nitrophenol isomer samples with five parallel measurements.



the satisfactory discrimination capability of proposed sensor array. For nitrophenol isomers analysis, the Au NCs sensor is simple, high-throughput and low-cost, with comparable or better sensing performance as compared to other quantum dots, carbon nanodots or metal nanoclusters, etc.5, 49-51 Taken all the data together, three β-CD-involved dual-ligands functionalized Au NCs sensor array have high differentiation capability to nitrophenol isomers.

Figure 4. EDs of the sensor array plotted against the different concentrations of PNP. Inset: the linear relationship of PNP concentration from 1 to 50 μM.

tested using the prosed Au NCs array. Interestingly, these mixtures, as well as pure PNP and ONP, were clearly distinguished from each other using first two canonical factors with excellent clustering and satisfied dispersion of 100% total variance (Figure 5a). And all of the 20 cases (4 isomers/mixtures × 5 replicates) were correctly assigned to their respective groups (Figure 5b). Similarly, mixtures from PNP/MNP or MNP/ONP were also well-distinguished and clustered into different groups. The resulting two dimensional canonical score plot (Figure S-20 and S-21, Supporting Information) showed clear clustering of the fluorescence response patterns using only the first two principal components with excellent discriminatory capacity (representing100% of the total variance). Meanwhile, using the same analytical method, interference on isomer sensing from possible interferents including nitrophenol derivatives and diphenols were investigated. Interestingly, these possible interferents did not cause interference to the differentiation of the three isomers (Figure S-22). This result suggests

Analysis of Blind Samples and Real Water Samples. The Au NCs sensor array can be chosen as working array for nitrophenol isomer differentiation because of its advantages mentioned above. To verify the detection accuracy of our nitrophenol isomer sensing array, a series of single isomer or two isomers mixture solutions (total concentration of 10 μM) were chosen as the unknown samples. Accordingly, the fluorescence response patterns were collected through the same approach and performed with LDA analysis. As can be seen from the Table S1 (Supporting Information), of the 51 unknown isomer samples, only one of them was incorrectly identified, affording an identification accuracy of 98.0% (The original data were shown in Table S1 in Supporting Information). The high identification accuracy further validated that this proposed Au NCs sensing array possesses convincing capability for the discrimination of nitrophenol isomers. And this sensor array may also capable for nitrophenol isomer analysis with real water samples since it shows satisfactory single isomer and two isomers mixtures discrimination ability. To investigate potential application of the designed system in real samples, we tested river water samples with spiked nitrophenol isomers. It was seen that the three isomers could be well discriminated at a concentration of 10 μM (Figure S-23). The first two canonical factors contained 69.7 and 30.3% of the variation, occupying 100% of the total variation. The well-separated fluorescence response patterns, as well as the high total variation of first two canonical factors, make the β-CD involved dual-ligands functionalized Au NC-based sensor array adaptable for nitrophenol isomer analysis in real water samples. Despite the no demand of specific interaction between surface ligand and target of sensor array, the introduction of selective recognition can effectively enhance the differentiation performance to similar targets with only tiny difference. Therefore, the combination of the “chemical nose/tongue” strategy with specific interaction may facilitate analysis of other complex isomer samples.

CONCLUSIONS

Figure 5. (a) Canonical score plots for the first two factors of relative fluorescence increase (I-I₀)/I₀ pattern from PNP/ONP mixture analyzed by LDA. (b) HCA analysis of mixture samples with five parallel measurements.

In summary, we have designed and synthesized three β-CD involved dual-ligands functionalized Au NCs with differentcolor emitting profiles and constructed a nitrophenol isomer sensor array based on those Au NCs. By integrating host-guest interaction and inner filter effect, the proposed sensor array successfully discriminate three nitrophenol isomers at concentration of 5 μM through LDA. Beyond satisfactory single isomer discrimination capability, mixture of two isomers can also be discriminated using this sensor array. The practical application of this sensor array is further validated by the high accuracy of blind sample tests and accurate distinction in river water real samples. Our study also demonstrates an interesting combination of “chemical nose/tongue” strategy and specific interaction, which increases the differentiation ability of sensor. And thus new avenues for the design of sensor array for other


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Analytical Chemistry isomer analytes with very small difference based on a similar strategy might be open up in the analytical and related fields.

ASSOCIATED CONTENT Supporting Information Additional figures and table as indicated in the main text. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Email: [email protected]. Phone: +86 10 64411957. Fax: +86 10 64411957. * Email: [email protected]. Phone: +86 10 64411957. Fax: +86 10 64411957.

ORCID

Zhiqin Yuan: 0000-0002-9715-4449 Chao Lu: 0000-0002-7841-7477 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21605003, 21656001, 21521005 and 21575010), the National Basic Research Program of China (973 Program, No. 2014CB932103), the China Postdoctoral Science Foundation (2016M600899), the Fundamental Research Funds for the Central Universities (buctrc201619).

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Fluorescent Gold Nanocluster-Based Sensor Array for Nitrophenol

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