CO2-Responsive Polymer-Functionalized Au Nanoparticles for CO2

Article pubs.acs.org/ac

CO2‑Responsive Polymer-Functionalized Au Nanoparticles for CO2 Sensor Ying Ma,*,† Kittithat Promthaveepong,† and Nan Li*,‡ †

Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 ‡ Division of Bioengineering, School of Chemical & Biomedical Engineering, Nanyang Technological University, 70 Nanyang Drive, Singapore 637457 S Supporting Information *

ABSTRACT: Metallic nanoparticles (NPs) coated with stimuli-responsive polymers (SRPs) exhibit tunable optical properties responding to external stimuli and show promising sensing applications. We present a new CO2-responsive polymer, poly(N-(3-amidino)-aniline) (PNAAN), coated gold NPs (AuNPs) synthesized by directly reducing HAuCl4 with a CO2-responsive monomer N-(3amidino)-aniline (NAAN). The amidine group of PNAAN can be protonated into a hydrophilic amidinium group by dissolved CO2 (dCO2). This induces the PNAAN to swell and detach from the AuNP surface, resulting in AuNP aggregation and color change. By monitoring the UV absorbance change of AuNPs, a sensitive dCO2 sensor with a linear range of 0.0132 to 0.1584 hPa and a limit of detection (LOD) of 0.0024 hPa is developed. This method shows dramatic improvement in sensitivity and convenience of sample preparation compared with the previously reported dCO2 sensor.

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negatively charged AuNP aggregation and color change through the electrostatic interaction in the presence of dCO2.24 Recently, we reported a CO2-responsive aniline (ANI) derivatives N-(3-amidino)-aniline (NAAN), which can be polymerized by ammonium persulfate (APS) into CO2responsive poly(N-(3-amidino)-aniline) (PNAAN) polymer.25 In this study, we found that HAuCl4 could also oxidize and polymerize NAAN into PNAAN, and HAuCl4 itself be reduced to form AuNPs with PNAAN capped on their surface as a stabilizer. Interestingly, the resulting AuNPs aggregated and changed their color from red to blue in the presence of dCO2 due to the swelling and subsequent detaching of the protonated PNAAN from AuNP surface. The AuNP color change provides a simple, sensitive method for dCO2 sensing in comparison with our previous method and reported method from other groups.

timuli-responsive polymers (SRPs) are high-performance polymers that change their conformations on external stimuli such as temperature,1 pH value,2 light, magnetic field,3 electricity,4 and solvent/water.5 These materials are playing increasingly important roles in a variety of applications such as bioadhesion,6−8 actuator,9,10 drug delivery,11 diagnostics, biosensors,12,13 and microelectromechanical systems.14 Nanoparticles, especially gold nanoparticles (AuNPs), exhibit distinctive, tunable optical properties, which makes them excellent scaffolds for sensor applications.15 The synergistic combination of polymers, especially SRPs with NPs has attracted great attention as the external stimulus can tune the SRPs conformation, which can in turn modulate the optical, electrical, and catalytic properties of NPs.16 For example, AuNPs coated with thermally responsive polymer poly(Nisopropylacrylamide) (PNIPAM) shows thermosensitive surface-enhanced Raman scattering (SERS) signal17 and tunable catalytic properties.18 CO2-responsive polymers has been extensively investigated recently as they have the merits of using CO2 as a “green” trigger compared with other thermal, pH, or light-responsive polymers. Additionally, CO2 can be removed by purging with an inert gas such as N2. This allows the reversible trigger of polymers without the accumulation of byproducts.19 Such polymers have been widely used in the fields of CO2 capture,20 CO2-switchable surface21 and surfactant,22 and CO2-responsive polymeric vesicles.23 We have reported a CO2-responsive copolymer poly(dimethyl acrylamide-co-(N-amidino) ethyl acrylamide), or P(DMA-co-NAEAA), which can induce the © XXXX American Chemical Society

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MATERIALS AND METHODS Chemicals and Reagents. HAuCl4·3 H2O was purchased from Sigma. NAAN was synthesized according to our reported method.25 All other chemicals are reagent grade and used without further purification. Ultrapure water was purified with a Millipore water system and purged with N2 for 15 min prior to use. Instruments and Measurements. UV−vis absorption spectra were measured by a Varian Cary60 spectrophotometer. Received: May 31, 2016 Accepted: July 12, 2016

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DOI: 10.1021/acs.analchem.6b02133 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry AuNP morphologies were characterized by a Transmission electron microscopy (TEM) (JEOL JEM 2100F TEM). The size of the AuNPs and standard deviation were calculated by measuring over 200 individual particles. Dynamic light laser scattering (DLS) spectra were measured with a Malvern DLS Zetasizer Nano S instrument. X-ray photoelectron spectroscopy (XPS) data was collected using a Kratos Axis Ultra DLD spectrometer (Kratos Analytical Ltd.) equipped with a monochromatized Al Kα X-ray source (1486.71 eV photons). Synthesis of PNAAN-Capped AuNPs. AuNPs were synthesized via the wet chemistry method. Simply, 4 μL of 10% HAuCl4 (11.8 mM) was injected into 1 mL of 10 mM NaOH aqueous solution and different concentrations of PVP (32 mg/mL) were subsequently added. After being mixed by a vortex for 30 s, the mixture (solution 1) was cooled to 4 °C. In another vial, NAAN was dissolved in water (400 mg/mL) and cooled to 4 °C as well (solution 2). Different volumes of solution 2 were injected into solution 1, and the reactants were stirred for 10 min. The acquired products were purified by a centrifuge/disperse procedure three times and finally dissolved in water for characterization. The AuNP concentration was calculated by UV absorbance with the Beer−Lambert Law according to the reported method.26 CO2-Responsive Properties of AuNPs. To study the CO2-responsive properties, AuNP solution was purged with CO2 gas for 15 min, and its morphology was characterized by TEM. To quantitatively measure the dCO2 concentration, NaHCO3 solution was used as a standard sample. Initially, 697.2 μL of the AuNP solution was mixed with 2.8 μL NaHCO3 stock solution with concentrations ranging from 0.0125 to 1 M. After incubation of the mixture for 10 min, the UV−vis absorption spectra of corresponding samples were recorded.

Figure 1. (a−c) TEM images of AuNPs synthesized in the presence of PVP concentrations of (a) 0.4, (b) 0.8, and (c) 1.6 mg/mL, respectively. The concentrations of NAAN and HAuCl4 are 16 mM and 11.8 mM, respectively. (d−f) TEM images of AuNPs synthesized in the presence of NAAN concentrations of (d) 8, (e) 16, and (f) 32 mM. The concentrations of HAuCl4 and PVP are 11.8 mM and 0.8 mg/mL, respectively. Average particle size of (g) sample a−c and (h) sample d−f (error bar = standard deviation).

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Figure 2. UV−vis absorption spectra of corresponding AuNP samples shown in Figure 1.

RESULTS AND DISCUSSION Synthesis of PNAAN-Capped AuNPs. NAAN is an amidine-carrying aniline (ANI) derivative. Similar to ANI, it can reduce HAuCl4 to Au atoms which grow into AuNPs,27,28 while NAAN itself is oxidized to form PNAAN. The resulting PNAAN binds to the AuNP surface as a capping agent. After injection of NAAN into HAuCl4 solutions containing PVP concentrations from 0.4, 0.8, and 1.6 mg/mL, the solution colors for all these cases changed from colorless to yellow and red in shorter than 1 min. After 10 min of incubation, TEM images of the resulting samples show spherical AuNPs with PVP concentration-dependent size variation (Figure 1a−c). A lower concentration of PVP (0.4 mg/mL) led to relatively larger AuNPs (47.6 ± 3.0 nm), while increasing the PVP concentrations to 0.8 and 1.6 mg/mL generated smaller AuNPs (35.3 ± 3.2 and 31.6 ± 2.4 nm for PVP concentration of 0.8 and 1.6 mg/mL, respectively). The corresponding AuNPs show typical surface plasmon resonance (SPR) absorption and the peak positions shift to shorter wavelengths as the PVP concentrations increase (Figure 2a), which is consistent with the report that a decrease in AuNP size results in a blue shift of the UV absorption peak.26 The ratio of NAAN/HAuCl4 affects the AuNP morphology and size as increasing the concentration of the reducing agent accelerates the reduction reaction of the HAuCl4. At a fixed HAuCl4 (11.8 mM) and PVP (0.8 mg/mL) concentrations, increase of NAAN concentration from 8, 16, 32 mM resulted in the increased AuNP size from 29 ± 5.7, 35.3 ± 3.2 to 36.9 ± 5.2 nm, respectively (Figure 1d−f). AuNPs synthesized using

16 mM NAAN as reducing agent exhibited a narrow SPR absorption peak, while lower or higher NAAN concentrations induced AuNPs with broad absorption peaks (Figure 2b). To identify the role of the amidine group in the AuNP formation, a control experiment was conducted using ANI in place of NAAN. Unlike the red color AuNPs, some precipitates were observed after 10 min when ANI was used as a reducing agent. Their corresponding SEM image reveals the aggregated AuNPs cross-linked by polyaniline (PANI) (Figure S1a), which is consistent with a weak and broad UV absorption peak at 500−700 nm (Figure S1b). This observation suggests that the amidine group at meta-position of ANI plays an important role in the AuNP formation. First, the presence of the amidine group decreases the polymerization rate of NAAN due to the steric hindrance and thus the reduction rate of HAuCl4. Second, the relative hydrophilic property of PNAAN prevents the aggregation of AuNPs, allowing them to grow into individual AuNPs. However, the formed AuNPs using ANI as a reducing agent are prone to aggregate immediately after their formation owing to the strong π−π stacking and hydrophobic interactions.29 CO2-Responsive AuNPs. The key merit of the amidine group is its ability to capture the H+ released by dCO2 and be protonated into amidinium.30 Once protonated by dCO2, PNAAN polymer becomes more hydrophilic and swelling occurs.25 Upon purging with gas CO2, the color of AuNP1 solution (AuNP synthesized using 0.8 mg/mL PVP and 16 mM B

DOI: 10.1021/acs.analchem.6b02133 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry NAAN) gradually changed from red to blue (inset of Figure 3a,b, respectively), revealing the aggregation of AuNPs that was further confirmed by TEM image (Figure 3b).

Figure 4. (a) Photographs, (b) UV absorption spectra, and (c) size distribution of (I) AuNP solution and AuNPs after incubation with 2 mM (II) NaHCO3, (III) NaCl, and (IV) Na2CO3. Figure 3. TEM images of AuNPs (a) before and (b) after being purged with CO2 gas. (c) Schematic representation of AuNP aggregation induced by dCO2. Inset of a and b: the photos of AuNP solution (a) before and (b) after being purged with CO2 gas.

AuNPs measured by TEM image (35.3 nm). A broad peak with the maximal peak intensity located at 427.1 nm was observed for sample II (II in Figure 4c), representing the aggregation of AuNPs. Different concentrations of NaHCO 3 solution were incubated with the AuNP solution to evaluate the performance of dCO2 assay. As the NaHCO3 concentration increased, AuNP solution color changed from red to purple and finally gray (Figure 5a), indicating the dCO2 concentration-dependent

The possible AuNP aggregation mechanism is proposed in Figure 3c: PNAAN polymer is neutral at the synthetic conditions, and it can physically adsorb on the AuNP surface as a stabilizer. CO2 + H 2O ≡ H+ + HCO−3

After purging with CO2, the equilibrium of CO2 with water is established to form H+ and HCO3−1 as shown in the following equation.31 The resulting H+ protonates the neutral amidine group into a more hydrophilic amidinium group, thus inducing the PNAAN polymer to swell and detach from AuNP surface. This leads to a decreased amount of stabilizer on AuNP surface and the AuNP aggregation. Colorimetric Detection of dCO2. To quantitively measure dCO2 concentration, NaHCO3 solution was used as an indirect standard sample and the corresponding H2CO3 or PCO2 concentration can be calculated according to previous work.32,33 After incubation with 2 mM NaHCO3 solution, the AuNP solution color changed to blue with some precipitates visible at the bottom of vial (II in Figure 4a), illustrating the aggregation of AuNPs. No visible color change was observed after incubation with the same concentration of NaCl solution (III in Figure 4a), indicating that the salt effect is not the cause of the AuNP aggregation, although it was reported to induce AuNP aggregation at high concentration.34 Moreover, AuNP solution shows similar red color after incubation with 2 mM Na2CO3 solution (IV in Figure 4a), which contains sodium and carbonate ions without H+ release. These observations demonstrate that the aggregation of AuNP is due to the H+ released by dCO2. The UV absorption spectra of corresponding samples (Figure 4b) show similar SPR absorption peaks for sample III and IV compared with original AuNP solution (sample I), while a broad peak was observed for sample II. DLS spectra of samples I, III, and IV exhibit narrow, symmetric peaks with the average AuNP size of 36.2, 36.5, and 37.2 nm respectively (Figure 4c), which is consistent with the size of well-dispersed

Figure 5. (a) Photographs and (b) UV absorption spectra of 0.22 nM AuNP1 solutions after incubation with 0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0 mM NaHCO3. (c) Plots of AuNP absorbance ratio (A680/ A534) vs NaHCO3 concentration.

aggregation of AuNPs. The UV absorption spectra of corresponding samples show the decreased absorbance at 534 nm and increased absorbance at 600−800 nm as NaHCO3 concentrations increased (Figure 5b). The absorbance ratio of A680/A534 linearly increased with NaHCO3 concentration between 0.05 and 0.6 mM (Figure 5c). The limit of detection (LOD) was calculated to be 0.009 mM by the equation of 3σ/ slope. The corresponding linear range for pCO2 concentration was calculated to be 0.0132 to 0.1584 hPa with a LOD of 0.0024 hPa according to the reported conversion calculation.32 C

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Analytical Chemistry

and NAAN. Element O is from PVP, and both PVP and PNAAN contain element C and N. In addition, the O/Au atomic ratio increases from 0.339 to 0.483 for AuNP1 and AuNP2 (AuNPs synthesized using 1.6 mg/mL PVP and 16 mM NAAN), indicating that AuNP2 has higher PVP coverage. After incubating AuNP2 (0.22 nM) with different concentrations of NaHCO3 solution, no obvious color changes were found when NaHCO3 concentrations are below 0.8 mM (Figure 7a), and their corresponding UV absorption spectra

This LOD is much lower than the reported method using solvatochromatic probe (LOD of 1.5 hPa)33 and surface plasmon resonance (SPR) as an detector (LOD of 6.8 hPa).35 It also shows greatly improved performance compared with our reported method (linear range of 0.264−6.336 hPa, and LOD of 0.04 hPa).24 Moreover, this sensor displays great advantages over the reported methods: (1) It is easy to prepare the sensing probe (one-pot reaction in 10 min); (2) It is a reagentless sensor (no additional agents are required for dCO2 sensor); (3) The measurement is simple (with bare eyes or by UV absorption spectrometer). Thus, we believe that this sensor may show promising application in environmental monitoring. To explore the effect of the AuNP concentration on the dCO2 assay, the sensing ability of a diluted AuNP solution (0.11 nM AuNP) was studied. After incubation with different concentrations of NaHCO3, the AuNP solutions show similar color and UV absorbance changes compared with a concentrated AuNP solution (0.22 nM) (Figure 6a,b). A

Figure 7. (a) Photographs and (b) UV absorption spectra of 0.22 nM AuNP2 solution in the presence of different concentrations of NaHCO3. (c) UV absorption spectra of AuNP1 and AuNP2 solution in the presence of 0.8 mM NaHCO3.

show a slight absorbance decrease at 534 nm (Figure 7b). In comparison, the UV absorption spectra of AuNP1 solution (0.22 nM) exhibited an obvious absorbance change under the same condition (Figure 7c). This is consistent with our assumption. Synthesis in the presence of even lower PVP concentrations may lead to AuNPs capped with more PNAAN and less PVP, which can show higher sensitivity for the dCO2 assay. However, their size-distributions increase and corresponding SPR absorption bands become broader as we discussed in AuNP synthesis section, which is a drawback for colorimetric detection. Thus, AuNP1 was selected as the optimal sample for the dCO2 assay.

Figure 6. (a) Photographs and (b) UV absorption spectra of 0.11 nM AuNP1 solution after incubation with different concentrations of NaHCO3. (c) Plots of AuNP absorbance ratio (A680/A534) vs NaHCO3 concentration for 0.22 and 0.11 nM AuNPs.

similar linear range between 0.05 and 0.6 mM NaHCO3 with correlation coefficient of 0.990 is shown in Figure 6c. However, the slope of 0.11 nM AuNP sample is smaller than that of 0.22 nM AuNPs, revealing a lower sensitivity for lower AuNP concentration. The reason is that the aggregation rate of AuNPs is dependent on the collision frequency of the particles, which decreases as the AuNP concentration decreases.36 Although even higher AuNP concentrations may exhibit a sensitive dCO2 assay, 0.22 nM AuNP solution (with maximal absorbance of 1.03) was selected as an optimal concentration because the optimal linear range between absorbance and concentration holds between 0.1 and 1.5 according to the Beer−Lambert law. Because PNAAN and PVP coadsorb on AuNP surface as capping agents, synthesis in the presence of higher PVP concentrations can result in AuNPs capped with more PVP and less PNAAN. This may decrease the dCO2 sensing performance as even when PNAAN is detached from the AuNP surface by dCO2, the residual PVP can still stabilize them and prevent them from aggregating. Thus, more PVP is unfavorable for the assay sensitivity. The chemical composition of AuNPs was characterized by XPS spectra (Figure S2). The presence of element C, N, O, and Au reveals the AuNPs coated with PVP

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CONCLUSIONS In summary, we have successfully synthesized CO2-responsive polymer (PNAAN) coated AuNPs by directly reducing HAuCl4 with a CO2-responsive monomer. The resulting AuNPs exhibited good performance for dCO2 sensing compared with reported methods. This method greatly improved the sensitivity and LOD. More importantly, it was found to be simple both in the AuNP synthesis and dCO2 sensing. For AuNP synthesis, it is a one-step, low-temperature reaction where the whole process only takes 10 min. For the dCO2 sensing, it is simple because both the sensor probe (PNAAN) and indicator (AuNPs) are incorporated into one unit, and no additional reagents are required for the assay.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b02133. D

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SEM image and UV absorption spectra of AuNPs using ANI as a reducing agent and the XPS data of AuNPs (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by research funding from the Singapore Millennium Foundation and the National Medical Research Council for funding support (NMRC/CIRG/1358/ 2013).

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