Highly Sensitive Fluorescent and Colorimetric pH Sensor Based on

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Highly Sensitive Fluorescent and Colorimetric pH Sensor Based on Polyethylenimine-Capped Silver Nanoclusters Fei Qu, Nian Bing Li,* and Hong Qun Luo* Key Laboratory of Eco-environments in Three Gorges Reservoir Region (Ministry of Education), School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, P. R. China S Supporting Information *

ABSTRACT: Silver nanoclusters capped by hyperbranched polyethylenimine (PEI) have been developed as a highly sensitive fluorescent and colorimetric pH sensor. The probe responds rapidly to pH fluctuations and has such absorption characteristics that the color changes from the colorless or a nearly colorless state to a colored state with increasing acidity, so PEI-capped Ag nanoclusters could be used as a color indicator for colorimetric pH detection. Quantitatively, the fluorescence intensity of PEI-capped Ag nanoclusters exhibits a linear fashion over the pH range of 5.02−7.96 and increases by around 10-fold approximately with greater fluorescence at higher pH values. The repulsion development and conformational change of PEI with decreasing pH induce the aggregation of Ag nanoclusters, leading to an obvious color change and fluorescence quenching of Ag nanoclusters at low pH values. As expected, the pH probe is also sensitive to the different buffer solutions, except for those containing some anions that could react with Ag nanoclusters. Besides, the ionic strength of the buffers has a little influence on the pH-responsive behavior. Our pH sensor with nanoscaled physical dimensions would be a promising candidate in the applications in biological, medical, and pharmaceutical fields.

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dots (QDs)9−14 and modified nanoparticles.15−21 However, few studies have been made in the pH-responsive optical properties of nanoclusters. Herein, for the first time, Ag nanoclusters capped by hyperbranched polyethylenimine (PEI) are employed as an effective fluorescent and colorimetric pH sensor, which is intrinsically pH sensitive, requiring no further functionalization. The most prominent feature of PEI is its high cationic charge density for every third atom is a nitrogen atom capable of protonation (Figure S1). It is modeled at three protonation levels: high pH (>10) with PEI fully deprotonated (uncharged), neutral pH (∼7) with all primary amines protonated, and low pH ( 8. Subsequently, with further decreasing pH, the absorbance around 400 nm increases gradually; finally, a characteristic surface plasmon resonance band attributed to larger Ag nanoparticles appears at 405 nm in acid medium,33−37 resulting in an obvious color change. Because of the progressive aggregation of Ag nanoclusters, the fluorescence of PEI-capped Ag nanoclusters gradually decreased with the reaction time in the pH linear range. However, the instability of the fluorescence seemed not to influence the sensitivity of pH responsive behavior (Figure S2). According to Figure S2b, Table S1 lists the linear equation, linear range, and the correlation coefficient R with the different reaction time. It could be found that the slopes of pH calibration curves of the different reaction time were similar, and the fluorescence intensities still increased by around 10fold approximately in the linear ranges. Besides, the ionic strength also has a little effect on the pH sensing action. The obtained PEI-capped Ag nanoclusters exhibited a slight instability under high ionic strength, resulting in the changes in fluorescence intensities. The sensitivity of pH response would be decreased slightly with increasing ionic strength. In spite of this, the ionic strength would not affect the pH-responsive action (Figure S3). All of these afford significant advantages to the PEI-capped Ag nanoclusters for the applications in various environments. As expected, the pH sensor is also sensitive to the different buffer solutions, such as acetate buffers, borax buffers, tris(hydroxymethyl)aminomethane (Tris)−CH3COOH buf-

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RESULTS AND DISCUSSION In this paper, a modified one-pot method based on PEImodified silver mirror reaction is used to synthesize Ag nanoclusters,28,29 which exhibits a fluorescence quantum yield of 3.8% in ethanol calculated by use of quinine sulfate as a reference.30−32 The water-soluble and blue-emitting PEIcapped Ag nanoclusters exhibit an average diameter of ∼1.8 nm shown in transmission electron microscopy (TEM) image (Figure 1a) and could keep stable for several months without any obvious colloidal aggregation. Figure 1b depicts the ultraviolet−visible (UV−vis) and fluorescence spectra of the diluted solutions of silver nanoclusters in water. As shown, an optical absorption feature is at 354 nm (curve 1) matched with the maximum fluorescence excitation (curve 2) and emission (curve 3) wavelengths of 375 and 455 nm, respectively. The diluted solution of PEI-capped Ag nanoclusters in water is nearly colorless (or a very slight yellow color) under visible light, while it emits intense blue fluorescence under a UV lamp (inset of Figure 1b). The standard sensing action of PEI-capped Ag nanoclusters is characterized by fluorescence and UV−vis spectra in BrittonRobinson (BR) buffer solutions from pH 11.98 to 1.81. As shown in Figure 2, the PEI-capped Ag nanoclusters are highly sensitive to the pH values adjusted by BR buffers. The fluorescence intensity reaches the maximum value and keeps B

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Figure 2. (a) Fluorescence spectra of PEI-capped Ag nanoclusters in BR buffers from pH 9.15 to 1.81 and (b) from pH 3.78 to 1.81. (c) Variation of emission wavelengths (λem) and fluorescence intensities of PEI-capped Ag nanoclusters in BR buffers at different pH values. The inset of (c) shows the calibration curve from pH 5.02 to 7.96.

Figure 3. Photographs for recording the color changes of PEI-capped Ag nanoclusters in BR buffers at different pH values (1.81−11.58) under visible light (a) and UV light (b).

Figure 5. The pH responses of fluorescence intensities of PEI-capped Ag nanoclusters in different buffer solutions: 1, BR buffers; 2, CH3COOH−CH3COONa buffers; 3, NaH2PO4−Na2HPO4 buffers; 4, citrate acid−sodium citrate buffers; 5, Na2BO4−NaOH buffers; 6, H3BO3−Na2BO4 buffers; 7, Tris−CH3COOH buffers; 8, HEPES− NaOH buffers; 9, Tris−HCl buffers.

Figure 4. UV−vis spectra of PEI-capped Ag nanoclusters in BR buffers at different pH values.

fers, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)−NaOH buffers, and so on. The color of the reaction solutions, fluorescence, and UV−vis spectra also presented the regular changes with decreasing pH (Figures S4−S9). Good corresponding data are obtained between the pH values calculated by the calibration curve and those measured by pH meter (Figure 5), indicating that the sensing actions are owing to the variation of pH values rather than the reaction between Ag nanoclusters and ions contained in the buffer solutions. As the buffer capacity is in the pH range from 7.0 to 12.0, such as

HEPES buffers, borax buffers, and Tris-CH3COOH buffers, the PEI-capped Ag nanoclusters also exhibit stronger fluorescence at higher pH values but without obvious color changes and variations in the UV−vis spectra. Moreover, it is important to notice that Ag nanoclusters would react with some anions contained in the buffer solutions, such as chloride ions, C

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Figure 6. (a) Protonation degree (α) of PEI and PEI-capped Ag nanoclusters (AgNCs) as a function of pH in the absence and presence of 0.1 M KNO3. (b) Zeta (ζ) potential of PEI and PEI-capped Ag nanoclusters (left) and hydrodynamic diameter (HDD) of PEI-capped Ag nanoclusters (right) in 0.1 M KNO3 at different pH values.

stability of Ag nanoclusters. However, at acidic pH values, both PEI and PEI-capped Ag nanoclusters possess considerable positive charges, leading to an expansion of PEI chains because of the repulsion between the charged amine groups. The charge−charge repulsions may be minimized by forcing the charged moieties as far apart as possible. Such pH-induced conformational changes have been reported for linear PEI, poly(propylenimine) (PPI), and NH2-terminated polyamidoamine (PAMAM) dendrimers.42−45 Therefore, the fluorescence quenching of PEI-capped Ag nanoclusters at lower pH values may result from two aspects: on the one hand, the Ag nanoclusters aggregate due to the loss of ligand protection resulting from the protonation of amines of PEI; on the other hand, the expansion in both of the intramolecular and intermolecular spaces because of the intense Coulombic repulsion caused by the multiple positive charges residing on each segment of PEI gives rise to a direct exposure of the nanoclusters to the solutions, which also contributes to the rapid aggregation of Ag nanoclusters. In other words, pHinduced conformational changes of PEI and the subsequent aggregation of Ag nanoclusters result in an obvious color change and fluorescence quenching at low pH values. Moreover, an alternative explanation for these observations is the distortion of the emission by reabsorption effect. If this is the case, some or all of the photons emitted by the fluorophore may be absorbed again. The result is that the intensity of the excitation light is not constant throughout the solution, and thus the fluorescence is quenched. Therefore, we compared the absorption spectra with emission spectra at different pH values in detail (Figure S12), such as pH 1.81, 2.36, 3.78, 5.02, 7.54, and 10.38. However, along with the decreasing pH from basicity to acidity, the emission band explicitly shows a red-shift from 455 to 474 nm and the absorption band also presents a red-shift from 354 to 405 nm. In this process, there is a small overlap between the absorption and emission bands (Figure S12c,d), which could not be avoided. In spite of this, the reason for the fluorescence quenching is mainly because of the formation of large nanoparticles, rather than the reabsorption effects. Additionally, the aggregated silver nanoclusters or large particles would be observed to dissolve with time in acid medium, especially in strong acidity and high concentrations of electrolyte solution. For example, in BR buffer solution at pH 1.81, the yellow color of Ag colloid solution faded gradually even to colorless, corresponding with the decreasing absorbance around 400 nm and slight changes in fluorescence spectra (Figures S13 and S14). The dissolution behavior is another piece of evidence that silver nanoclusters lose the

phosphate ions, and citrate ions, resulting in deviations in the pH determination. Meanwhile, the color of solutions deepens significantly over time due to the reaction between Ag atoms and anions mentioned above (Figures S10 and S11). The pH sensitivity of Ag nanoclusters to the external environments arises in part from the ligand’s role of the template of PEI, which significantly contributes to the surface chemistry and physical characteristics of the nanoclusters. This is expected to have pronounced consequences for the mechanism of pH response. The standard method of determining the surface charge of PEI with the variation of pH is based on the acid−base titration.22,38 The overall protonation degree (α) of PEI is plotted as a function of pH in the absence and presence of electrolyte solution (0.1 M KNO3), and the protonation degree of PEI-capped Ag nanoclusters was also titrated for the first time (Figure 6a). The titration (or charge density) curves show the characteristic dependence with the pH and ionic strength for PEI as a polybase, which is becoming more acidic with decreasing ionic strength:39 the point of zero charge of PEI is approximately at pH 9.92 without salt and is about 10.35 in 0.1 M KNO3 solution. The above behavior of PEI samples is in excellent agreement with that presented by previous reports.22,23 However, the nanoclusters are uncharged at pH 7.05−8.54 in the absence and presence of electrolyte solution, implying that most of amino groups of PEI are occupied by Ag nanoclusters. Such distinctive characteristics of PEI and PEI-capped Ag nanoclusters could also be illustrated by zeta (ζ) potential and hydrodynamic diameter (HDD) measurements (Figure 6b). PEI is positively charged over a broad pH range because the pKa values of its different amines are pH ∼9 for primary, pH 8 for secondary, and pH 6−7 for the tertiary amino group.17,40 However, the PEI-capped Ag nanoclusters appear an isoelectric point (IEP) at about pH 9.0 in 0.1 M KNO3 solution, so Ag nanoclusters are positively charged as pH < 9.0. Correspondingly, the increasing HDD with decreasing pH revealed that larger Ag nanoparticles or aggregated Ag nanoclusters formed in acid medium. It should be noticed that the apparent HDD of PEI-capped Ag nanoclusters (∼3.5 nm) is larger than the TEM diameter (Figure 1a) because dynamic light scattering (DLS) gives the size of solvent-swollen aggregates, whereas TEM gives the size of dry particles.41 When the pH is higher than IEP of the nanoclusters in buffer solutions, the high local concentrations of amine groups introduced by PEI could effectively chelate with silver atoms, instead of protonation. It means that there is little or no electrostatic repulsion between branches of PEI. The polymer chains are tightly coiled as a dense core, which could favor the D

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Zeta potential and hydrodynamic diameter measurements were performed by laser Doppler electrophoresis using a Zetasizer Nano ZS (Malvern Instruments Ltd., U.K.). The pH values of solutions were measured with a pH meter (PHS-3C, Shanghai Leici Instrument Company, Ltd., China). Preparation of PEI-Capped Ag Nanoclusters. Typically, 100 μL of 0.098 g mL−1 PEI and 50 μL of HEPES solution (1 mM) were first dissolved in deionized water (95 μL) by stirring for ∼2 min, and then 250 μL of 100 mM AgNO3 was added and homogenized by stirring for ∼2 min. Subsequently, 5 μL of formaldehyde solution (35 wt %) was added under vigorous stirring, and the mixture was heated at 70 °C for 10 min. The final solution was stocked at ambient environment for at least 48 h before its further application. Without any centrifugation or purification, the water-soluble and blue-emitting PEI-capped Ag nanoclusters were used to explore the response of pH in different buffer solutions. pH Response Experiments. Briefly, 200 μL of buffers, 790 μL of water, and 10 μL of PEI-capped Ag nanoclusters were mixed together with reaction time of 10 min, and then the fluorescence and UV−vis absorption spectra of the mixtures were recorded by fluorescence and UV−vis spectrophotometers. The excitation wavelength was 375 nm in all of the experiments in this paper. Influence of Ionic Strength on the pH Response of PEICapped Ag Nanoclusters. Typically, 200 μL of BR buffers, 690 μL of water, 100 μL of KNO3 with different concentrations, and 10 μL of PEI-capped Ag nanoclusters were mixed together with reaction time of 10 min, and then the fluorescence spectra of the mixtures were recorded by a fluorescence spectrophotometer. Typical Acid−Base Titration. The samples were prepared by diluting 50 μL of PEI-capped Ag nanoclusters (or 50 μL of 2.00 wt % PEI solution) to 5 mL of water in the absence and presence of 0.1 M KNO3, and then the samples were adjusted with HNO3 (0.1 M) and NaOH (0.1 M) by a pH meter to different pH from 2.0 to 11.0, respectively. All of the samples contained the same concentration of PEI (0.02 wt %). The degree of protonation (α) is calculated from the measured pH via

protection of PEI at lower pH, since bare silver particles are more sensitive to the presence of oxygen to produce a layer of Ag2O, which would dissolve with time due to electrolyteinduced perturbations and oxidative corrosion. Such dissolution is highly dependent on the electrolyte type and concentration.46 Nevertheless, this pH sensor is still irreversible in acid medium because the protonated PEI loses the ability of effective chelation of silver atoms. Finally, we attempted to investigate the contributions of three types of amines of PEI to the pH-sensitive behavior. Crooks et al. found that the pH-dependent behavior was directly related to the interior tertiary amine groups (pKa ∼ 6.3) of PAMAM dendrimers, according to the changes of an absorption band in the range 280−285 nm.47 The absorbance of PEI-capped Ag nanoclusters at 268 nm also declined with decreasing pH, but the pure PEI was absence of absorption band in this range (Figure S15). However, the tertiary amine of PEI is also considered to play a central role in the pHresponsive behavior because only its pKa locates in the pH linear range (from 5.02 to 7.96).

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CONCLUSIONS In summary, our characterization of the pH-dependent fluorescence of Ag nanoclusters shows how nanoscaled materials indicate pH values in various buffer solutions. Moreover, it could be used as a color indicator for colorimetric pH detection in the physiological environments (pH 5.0−7.4) because the color changes from the colorless or a nearly colorless state to a colored state with increasing acidity. The highly sensitive pH response of PEI-capped Ag nanoclusters is due to not only the charge distribution of different amine groups of PEI as a function of pH but also the change of the chain conformation on the local structure of the polymer. In other words, the electron donation capability of PEI along to Ag nanoclusters would also depend subtly on the medium pH. Finally, the PEI-capped Ag nanoclusters would be a promising candidate in the applications in biological, pharmaceutical, industrial fields, and so on.

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α=

[PEIH+] [PEI]total

(1)

[H+]total − [H+]free = [PEIH+]

(2)

where [PEI]total and [H+]total are the total concentrations of PEI and added acid, respectively, and [H+]free is the concentration of acid determined by the pH measurement. Dynamic Light Scattering Measurements. The zeta (ζ) potentials of PEI and PEI-capped Ag nanoclusters and hydrodynamic diameter (HDD) of PEI-capped Ag nanoclusters were examined by dynamic light scattering (DLS) as a function of pH. The DLS samples were prepared as follows: 50 μL of PEI-capped Ag nanoclusters (or 50 μL of 2.00 wt % PEI solution) were diluted in 5 mL of electrolyte solutions (0.1 M KNO3), and then the samples were adjusted with HNO3 (0.1 M) and NaOH (0.1 M) by pH meter to different pH from 2.0 to 11.0, respectively. All solutions used to detect HDD by DLS were passed through Millipore filters with a pore size of 0.22 μm to remove dust.

EXPERIMENTAL SECTION

Materials. Silver nitrate (AgNO3), hyperbranched polyethylenimine (PEI) (Mw 10 000, 99%), 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), and formaldehyde (35 wt %) were purchased from Aladdin Reagent Co., Ltd., China. All solutions were freshly prepared before use. Britton-Robinson (BR) buffers (pH 1.81− 11.98) were prepared by mixing 0.2 M NaOH and a mixture of 0.04 M H3PO4, H3BO3, and CH3COOH according to suitable proportion. All other buffer solutions were 0.1 M, including CH 3 COOH− CH3COONa buffers, NaH2PO4−Na2HPO4 buffers, Na2BO4−NaOH buffers, H3BO3−Na2BO4 buffers, tris(hydroxymethyl)aminomethane (Tris)−CH3COOH buffers, Tris−HCl buffers, HEPES−NaOH buffers, and citrate acid−sodium citrate buffers. All reagents used are of at least analytical reagent grade. Milli-Q water (18 MΩ cm) was used throughout the experiments. Instrumentation. Transmission electron microscopy (TEM) images were collected using a Hitachi 7500 transmission electron microscope (Japan) with an accelerating voltage of 80 kV. A diluted solution was spotted on carbon-coated copper grid and was dried in laboratory ambience. The pH response of PEI-capped Ag nanoclusters was characterized by a Hitachi F-4500 fluorescence spectrophotometer (Japan) with excitation at 375 nm in the emission mode. The slit width was 10 and 10 nm for excitation and emission, respectively. The photomultiplier tube (PMT) voltage was set at 400 V. UV−vis absorption spectra for colorimetric assay were recorded from 200 to 600 nm using a Shimadzu UV−vis 2450 spectrophotometer (Japan).

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ASSOCIATED CONTENT

S Supporting Information *

Responsive stability, the influence of ionic strength, fluorescence and UV−vis spectra of PEI-capped Ag nanoclusters in various buffers at different pH values, comparison of the absorption and emission spectra of PEI-capped Ag nanoclusters at different pH values, photographs for recording the color changes of PEI-capped Ag nanoclusters with time, and comparison of UV−vis spectra of pure PEI and PEI-capped Ag nanoclusters. This material is available free of charge via the Internet at http://pubs.acs.org. E

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (N.B.L.), [email protected] (H.Q.L.); Ph 86-23-68253237, Fax 86-23-68253237. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21273174, 20975083), the Municipal Science Foundation of Chongqing City (No. CSTC-2008BB 4013), and the 211 Project of Southwest University (the Third Term).

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