Recyclable Colorimetric Detection of Trivalent Cations in Aqueous

Mar 9, 2016 - Recyclable Colorimetric Detection of Trivalent Cations in Aqueous Media Using Zwitterionic Gold Nanoparticles. Wenshu Zheng†‡, Huan ...
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Recyclable Colorimetric Detection of Trivalent Cations in Aqueous Media Using Zwitterionic Gold Nanoparticles Wenshu Zheng,†,‡ Huan Li,§ Wenwen Chen,†,⊥ Jian Ji,*,§ and Xingyu Jiang*,† †

Beijing Engineering Research Center for BioNanotechnology & Key Lab for Biological Effects of Nanomaterials and Nanosafety, CAS Center for Excellence in Nanoscience, National Center for NanoScience and Technology & University of the Chinese Academy of Sciences, 11 Beiyitiao, Zhongguancun, Beijing 100190, China ‡ Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, China § MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: This report describes a colorimetric assay for trivalent metal cations (M3+) using gold nanoparticles (AuNPs)-modified with oppositely charged thiols that can form intermolecular zwitterionic surfaces. Zwitterionic AuNPs (Zw-AuNPs) are stable in high-salt solutions and welldispersed in a wide range of pH values. M3+ including Fe3+, Al3+, and Cr3+ can effectively trigger the aggregation of ZwAuNPs by interfering with their surface potential, and aggregated AuNPs can be regenerated and recycled by removing M3+. In our approach, the output signal can be observed by the naked eye within a micromolar (μM) concentration range. Uniquely, our assay is capable of discriminating Fe3+ from Fe2+, which is challenging using traditional approaches. More importantly, Zw-AuNPs can be stored stably at room temperature for a long period (3 months) with constant detection performance. Both the cost-effectiveness and the long shelf life make Zw-AuNPs ideal for detecting M3+ in resource-poor and remote areas.

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concentration and preservation behavior as well as the salt- and pH-resistant properties.22,23 Recent study shows that the surface potential of zwitterionic micelles in water is influenced by the valence of metal cations, and the effects of trivalent ions are much bigger than monovalent and divalent cations.24 Such valence-specific response would also apply to Zw-AuNPs due to their similar surface structures to that of zwitterionic micelles. We wondered if we could use Zw-AuNPs to assay for trivalent ions, given the fact that the dispersion state of AuNPs is closely related to the surface potential of AuNPs. Moreover, the optical and antiinterference properties of Zw-AuNPs allow them to be useful for sensing ions. Based on our previous work of AuNPs-based colorimetric systems,25−28 we attempt to utilize the specific response of zwitterionic surface and the optical property of AuNPs to explore a nanosensor for assaying M3+. The most common M3+, including Fe3+, Al3+ and Cr3+, are present and functional in the environment and living organisms. Fe3+, as an important transition metal ion in human, is critically involved in many biological processes such as electron transfer and oxygen transport.29 Cr3+ is an environmental pollutant that causes great

ven though gold nanoparticles (AuNPs) have inspired plenty of research interests due to their unique physicochemical properties,1,2 no report has demonstrated that AuNPs can selectively assay for trivalent metal ions (M3+) without interference from monovalent or divalent ions. This report shows that modification of AuNPs with oppositely charged molecules allows for selective detection of M3+. Surface modification of AuNPs introduces many new features and specific functions.3,4 For example, AuNPs capped with chemical ligands are used as nanosensors for pH sensing,5 temperature monitoring6 and other chemical and biological assays.7−9 Modified AuNPs can also be applied in bioimaging,10 cancer therapy11 and drug developments.12 Among the surface modification strategies for AuNPs, the zwitterionic surface is widely studied as an efficient way to improve the performance of AuNPs.13 Zwitterionic surface can be constructed from ligands that have a positively charged moiety and a negatively charged moiety in the same molecule, or 1:1 mixture of positively and negatively charged molecules.14,15 Once formed, they can protect solid surfaces against nonspecific protein adsorption and allow surfaces to be stable against a series of harsh conditions.16,17 The stability of AuNPs with a zwitterionic surface against protein adsorption is useful for both in vitro and in vivo research.18−21 Besides the antifouling features, AuNPs modified with a zwitterionic surface can exhibit pH-induced © XXXX American Chemical Society

Received: February 4, 2016 Accepted: March 9, 2016

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DOI: 10.1021/acs.analchem.6b00501 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry concern for health and agriculture.30 Al3+ is abundant in nature, but an excessive amount of Al3+ in food is harmful to humans.31 Thus, it is imperative to develop a simple, inexpensive, rapid, and reliable assay for M3+ in our daily life. Current assays like atomic absorption spectroscopy and inductively coupled plasma-mass spectrometry are considered as the gold standard for metal ions detection,32,33 but the requirements for both advanced instruments and skilled technicians limit their broad applications. In addition, these approaches fail to discriminate the same metal element with different valence, such as Fe3+ and Fe2+.34 Some organic fluorescent probes for analyzing M3+ are designed and synthesized recently,35,36 but these sensors cannot be directly applied to real samples due to one or several of these limitations: (i) poor water solubility; (ii) serious photobleach; (iii) requirement of rigorous operational conditions. By contrast, AuNPs with novel optical properties, particularly as the sensors for point-of-care assays, have resulted in many methods for detection of different chemical/biological targets.25−28,37−39 These assays are highly sensitive,37 instrument-free,38 and capable of multiple-target detection.39 However, when it comes to the issue of ion detection, most existing AuNPs-based assays, cannot selectively detect M3+ and are mostly focused on monovalent and divalent cations. Moreover, even though AuNPs-based assays are reported to be particularly used in resource-poor areas, the high cost of AuNPs as a product of a noble resource have made it a luxury to operate AuNPs-based assay in undeveloped regions. Furthermore, in most AuNPs-based assays, once AuNPs are aggregated, it is difficult to reuse them. Thus, an AuNPs-based assay for M3+ with the ability to be recycled would be of great significance. For analyzing these trivalent ions, sensitivity is less important to broad-spectrum response and valence specificity. In this work, we prepare Zw-AuNPs by the ligand exchange reaction with 1:1 molar ratio mixture of negatively charged sodium 10-mercaptodecanesulfonic acid and positively charged (10-mercaptodecyl)-trimethylammonium bromide, according to the previous report.40 Zw-AuNPs show valence-dependent color responses to M3+, whereas monovalent and divalent cations cannot cause any response (Scheme 1). On the basis of

strength. We also compare the performance of different sensors published for M3+ detection (Table S1).

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EXPERIMENTAL METHODS

Materials and Instruments. All chemical reagents were purchased from major suppliers such as Alfa Aesar, SigmaAldrich and used as received. Positive and negative charged thiols were synthesized and characterized according to the previous report.40 Tap water was collected from the National Center for NanoScience and Technology. Drinking water was bought from Beijing CSF market. Human urine was collected from a healthy nonsmoker at daytime. All glassware was cleaned by freshly prepared aqua regia solution (HCl/HNO3, 3:1). The UV−vis spectra were recorded with a UV2450 spectrophotometer (Shimadzu). Dynamic light scattering (DLS) and zeta potential (ζ) were performed on a Zeta Sizer Nano ZS (Malvern Zetasizer 3000HS and He/Ne laser at 632.8 nm at scattering angles of 90 deg at 25 °C). Transmission electron microscope (TEM) images were obtained by using a JEOL1400 model at an accelerating voltage of 200 kV. The photographs were record with a Nikon D90 camera. Preparation of 13 nm Citrate-Capped AuNPs. AuNPs (with their diameters ∼13 nm) were prepared by citratemediated reduction of HAuCl4 following a standard procedure. Briefly, an aqueous solution of HAuCl4 (1 mM, 100 mL) was heated to reflux with rapid stirring, and the preheating solution (95 °C) of sodium citrate (38.8 mM, 10 mL) was added rapidly. The color of the mixture changed from light yellow to red within 1 min. The resulting solution was boiled for an additional 15 min, cooled to room temperature and filtered through a 0.22 μm syringe filter, and stored in a refrigerator at 4 ° C for further experiments. The concentration of AuNPs was about 15 nM according to Beer’s law (extinction coefficient of 1.86 × 108 M−1 cm−1 at 520 nm for the 13 nm AuNPs). Preparation of Zw-AuNPs and S-AuNPs. Zw-AuNPs were obtained from citrate-coated AuNPs by a ligand exchange reaction. Briefly, a mixed thiols aqueous solution (10 mM, 2 mL) which contained a 1:1 ratio of sodium 10-mercaptodecanesulfonic acid and (10-mercaptodecyl) trimethylammonium bromide was added into citrate-coated AuNPs solution (10 mL, 15 nM). For sulfonic acid-capped AuNPs (S-AuNPs), only sodium 10-mercaptodecanesulfonic acid was added. The mixtures were stirred at room temperature for 24 h. After the centrifugations (three times), S-AuNPs could be used to detect cations. Experimental Procedure for the Detection of Metal Ions. Zw-AuNPs were prepared and diluted to 3.7 nM at pH 7, and then 10 μL different concentrations of a given metal ion solution were added to the solution of 1 mL Zw-AuNPs. The mixtures were mildly stirred for 5 min, and then the images and UV−vis measurements were recorded. All the measurements were repeated 5 times for each sample. Experimental Procedure for the Detection of Metal Ions in Real Samples. Zw-AuNPs were concentrated to 370 nM at pH 7 by centrifugation, and then of 10 μL Zw-AuNPs were added to the 1 mL samples. The mixtures were mildly stirred for 5 min before the images/UV−vis measurements were recorded. All the measurements were repeated 5 times for each sample.

Scheme 1. Zw-AuNPs Can Be Prepared by Coating AuNPs with Mixed Charged Thiols; M3+ Can Trigger the Aggregation of Zw-AuNPs

this principle, we developed a AuNPs-based system for M3+ assay, which has several advantages compared with existing methods: (i) high molar extinction coefficient of AuNPs enables the naked eye assay for M3+ with good sensitivity; (ii) Zw-AuNPs are easily stored and recycled, which can greatly save the cost and is environmental friendly; (iii) Zw-AuNPs are very stable in a wide pH range and tolerant of high ionic B

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Figure 1. Responses of Zw-AuNPs to different ions. (a) Color responses of Zw-AuNPs in the presence of 17 kinds of metal cations. The concentration for Cr3+, Al3+, Fe3+ is 25 μM; the concentration of the other ions is 1 mM; (b) (c) A650/520 value of Zw-AuNPs in the presence of different concentration of metal ions; (d) A650/520 values of Zw-AuNPs in the presence of different anions and ferric complexes.

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RESULTS AND DISCUSSION Colorimetric Detection of M3+ in Aqueous Solutions Using Zw-AuNPs. After the addition of any of the trivalent ions (i.e., Fe3+, Cr3+, and Al3+) to the solution of Zw-AuNPs, the color of Zw-AuNPs changed from red to blue or purple within 5 min, indicating the aggregation of AuNPs (Figure 1a). Transmission electron microscopy (TEM) shows that ZwAuNPs aggregated in the presence of M3+, whereas no aggregation occurred without M3+ (Figure S1a, S1b, Supporting Information). We used dynamic light scattering (DLS) to further confirm the aggregation of AuNPs by measuring the hydrated radius of Zw-AuNPs. The hydrodynamic ratio increased from 21 nm to 200−400 nm (Figure S1c, Supporting Information). These results indicated that M3+ can change the color of Zw-AuNPs by inducing their aggregation. Selectivity of Zw-AuNPs for the Detection of M3+. To test the selectivity of our assay, we added FeCl3, CrCl3, AlCl3, CaCl2, CuCl2, PbCl2, CdCl2, BaCl2, Hg(ClO4)2, CoCl2, FeCl2, SnCl2, MgCl2, ZnCl2, AgNO3, KCl, NaCl into Zw-AuNPs solution to obtain final concentration of these ions in the range between 1 μM and 100 μM. After the addition of different M3+, the absorbance value variation at 520 nm and at 650 nm can be easily observed (Figure S1d, Supporting Information). According to previous reports on AuNPs-based detection,25−28 the value of absorbance at 650 nm versus the absorbance at 520 nm (A650/520) was utilized to quantify the degree of aggregation for Zw-AuNPs, where a higher A650/520 indicates a greater degree of aggregation. The A650/520 value increase occurred only in the presence of Fe3+, Al3+, and Cr3+ (1−100 μM, Figure 1b and 1c). When we further increased the concentration of the other interfering metal cations to 1 mM, the color of Zw-AuNPs remained red (Figure 1a). To further confirm the valence-selective response, we tested the response of Zw-AuNPs to trivalent rare earth ions, such as Y3+ and Ce3+. In the presence of these cations (100 μM), the A650/520 value of Zw-AuNPs increased obviously (Figure 1d), suggesting that

these trivalent rare earth ions can also induce the aggregation of Zw-AuNPs. On the other hand, we tested whether or not compounds that contain M3+ but do not exist in a trivalent form in an aqueous solution, such as ferric pyrophosphate (Fe4(P2O7)3), iron thiocyanate (Fe(SCN)3), ferritin, and AlEDTA−, (Al = aluminum, EDTA = ethylenediaminetetraacetate), could trigger the aggregation of AuNPs even with a concentration of 1 mM (Figure 1d). These results indicate that Zw-AuNPs do not respond to complexes of M3+, as M3+ might include species like Fe4(P2O7)3 in the solution. We would also like to try cations with higher valence; however, we could hardly access higher valence metal cations that exist stably in aqueous solution as their original form. For example, hexavalent Cr would only exist as Cr2O72− in water and would not cause the aggregation of Zw-AuNPs (Figure 1d). These results also demonstrate that common metal ions other than M3+ cannot trigger the aggregation of Zw-AuNPs even at high concentrations. In order to study the influence of different anions on our assay, we incubated Zw-AuNPs with different sodium salts including NaCl, NaF, NaNO3, NaH2PO4, Na2SO4, Na2CO3, Na2HPO4, and Na3PO4 with a final concentration of 1 mM. Zw-AuNPs did not show any response to the anions with different valence (Figure 1d). All these results show that the selectivity of using Zw-AuNPs to detect free M3+ in aqueous samples is superb. Sensitivity of Zw-AuNPs for the Detection of M3+. To evaluate the limit of detection (LOD) for Fe3+ at neutral pH (pH = 7), we added FeCl3 into the solution of Zw-AuNPs to obtain the final Fe3+ concentrations ranging from 1 μM to 100 μM (Figure 2a). Using the naked eye only, we could detect the presence of Fe3+ above 2 μM. The color of Zw-AuNPs solutions changed gradually with the increasing concentration of Fe3+, which was consistent with the UV−vis spectra (Figure 2b). The linear range of values of A650/520 versus the Fe3+ concentration is between 1 μM and 50 μM (R2 = 0.998) (Figure 2c). Similar procedures were used to test the sensitivity C

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Range of pH for Effective Detection. Zw-AuNPs are dispersed well in a wide pH range (from 3 to 12), as demonstrated in the previous report.37 To show that ZwAuNPs could be stable in the presence of interfering ions, we added a mixture of 14 kinds of divalent and monovalent ions (Ca2+, Cu2+, Pb2+, Cd2+, Ba2+, Hg2+, Co2+, Fe2+, Sn2+, Mg2+, Zn2+, Ag+, K+, Na+, each with a final concentration of 100 mM) to the solution of Zw-AuNPs in a range of pH 3−12, while the color of the mixture still remained red. The result indicated ZwAuNPs can remain stable over a wide range of pH values without M3+. Next, we evaluated the pH influence on the response of Zw-AuNPs to M3+ by adjusting pH values between 3 and 12. Under alkaline conditions (pH > 10), M3+ could not trigger the color change of Zw-AuNPs, which is due to the formation of M(OH)3 as precipitates in the strong alkaline solution inhibit the binding between M3+ and Zw-AuNPs. Between pH 3 and 10, Fe3+, Al3+, and Cr3+ could all cause visible color change of Zw-AuNPs (Figure S3a, Supporting Information). We measured the A650/520 value of Zw-AuNPs in the presence of M3+ at different pH. The trend of the curves (Figure S3b, Supporting Information) matches with the different colors (Figure S3c, Supporting Information). These results indicated that the working range of this detection system includes a wide pH range. Recyclability and Stability of Zw-AuNPs. By adjusting pH from neutral to 11, the solution color of the aggregated ZwAuNPs with M3+ changed from purple to red, implying that the process of aggregation is reversible. This reversible aggregation would allow Zw-AuNPs to be recyclable for the detection of M3+. To illustrate the recycling process, we incubated the fresh Zw-AuNPs with 100 μM Fe3+ and recorded the increased value of A650/520, followed by adding NaOH solution (1 M) to adjust the pH value to 11. At pH 11, the color changed rapidly from purple to red, indicating the redispersion of Zw-AuNPs. We centrifuged the mixture, removed the supernatant and resuspended Zw-AuNPs in the aqueous solution. The redispersed Zw-AuNPs showed the same A650/520 value and response to Fe3+ as the original one. We here in confirmed that even after 3 rounds, Zw-AuNPs could still be used to assay Fe3+ (Figure 4a). The reason for the recyclability of Zw-AuNPs is on the basis of the different status of trivalent ions in the different pH solution. Under an acid or neutral environment, M3+ can interact with Zw-AuNPs and induce their aggregation. When the solution is adjusted to a strongly basic environment, free M3+ became M(OH)3 and inhibited the interaction between Zw-AuNPs and M3+, and thus, Zw-AuNPs redispersed. Besides the recyclability, the excellent stability of Zw-AuNPs is also critical for M3+ sensing. In most AuNPs-based sensing systems, it is difficult to keep AuNPs stable in aqueous at room temperature for a long period, and need to be prepared freshly to use or be stored in a refrigerator. For the Zw-AuNPs, even after being stored in room temperature for 3 months, ZwAuNPs showed the same performance for assaying M3+ as freshly synthesized ones (Figure 4b). The long-term stability of Zw-AuNPs is attributed the protection strategy of the zwitterionic surface assembled on AuNPs. The aforementioned two features, recyclability and long shelf life, endow our assay especially suitable for sensing M3+ in resource-poor settings. Interference for the Assay Zw-AuNPs and Their Application for Real Samples. To test the interference from the other metal ions, we tested the response of Zw-AuNPs to M3+ in the presence of the mixtures of other metal cations (mixtures containing all Ca2+, Cu2+, Pb2+, Cd2+, Ba2+, Hg2+,

Figure 2. Sensitivity of Zw-AuNPs for the detection of Fe3+. (a) Photos of Zw-AuNPs with different concentrations of Fe3+. (b) UV− vis spectra of AuNPs with different concentration of Fe3+ (from 1 μM To 100 μM). (c) Plot of A650/520 vs the concentration of Fe3+. Inset: the linear calibration plot of concentration-dependent responses A650/520 for Fe3+ detection. Linear range: 1 μM to 50 μM, R2 = 0.994.

for Al3+ and Cr3+. The LODs with the naked eye for Al3+ and Cr3+ are 10 μM and 18 μM, respectively (Figure S2, Supporting Information). We also explored the colorimetric response of Zw-AuNPs to the mixtures of Fe3+, Al3+, and Cr3+. Due to the lowest LOD for Fe3+, the colorimetric response in the samples with Fe3+ is more obvious than other M3+ (Figure 3). These results indicate that Zw-AuNPs show obvious and stable response to any one or a mixtures of different M3+ at low concentrations.

Figure 3. Photographs of Zw-AuNPs with mixtures of different M3+. The final concentrations of metal ions are listed under the photos. D

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Figure 4. Recyclability and stability of Zw-AuNPs for M3+ detection. (a) A650/520 values of aggregated (0.15) and redispersed Zw-AuNPs (0.65); (b) A650/520 values of Zw-AuNPs with Fe3+, Cr3+, and Al3+ measured in different storage time (1 day, 1 week, 1 month, 3 months). The concentration for Fe3+, Al3+, and Cr3+ are 25, 100, and 40 μM respectively.

Co2+, Fe2+, Sn2+,Mg2+, Zn2+, Ag+, K+, Na+) by measuring the A650/520 value. Even with a final concentration of 50 mM (more than 500 times of M3+) for these interfering ions, the responses of Zw-AuNPs were similar to the M3+ (Figure S4a, Supporting Information), indicating that other metal ions and high ionic strength have negligible interference on this detection system. We also tested the responses of Zw-AuNPs in the mixtures containing several representative small organic molecules. When organic molecules (1% methyl alcohol, 1% methyl toluene, and 1% acetone, vt %) coexist in the aqueous samples, Zw-AuNPs remained stable and showed the same A650/520 value responses after the addition of M3+ (Figure S4b, Supporting Information). Next, we evaluated the potential application of this system by applying Zw-AuNPs in drinking water tap water samples. Without M3+, these samples did not induce the color change of AuNPs (Figure 5a). By contrast, when M3+ ions were present in these samples, Zw-AuNPs aggregated with the color change from red to blue or purple (Figure 5a). To further confirm the utility of such a detection system in a real-world application, we attempted to detect M3+ in human urine samples. Zw-AuNPs are well-dispersed in healthy volunteers’ urine where free M3+ do not exist. However, a distinct color change can be observed when M3+ ions were added to and present in these samples (Figure 5a). These results confirm that Zw-AuNPs are robust, and can carry out sensing in environmental samples and biological matrixes without tedious pretreatments. Mechanism of the Detection of M3+. To understand the mechanisms of the M3+-caused aggregation of Zw-AuNPs, we test the response of a panel of ions to AuNPs modified only by negatively charged sulfonate terminated thiols nor modified only by the positively charged quaternary ammonium terminated thiols. We previously reported that the quaternary ammonium capped AuNPs (N-AuNPs) could detect mercury ions in aqueous media at neutral pH, which is attributed to the abstraction of thiols on the surface of AuNPs by Hg2+.26 In this

Figure 5. Real sample detection and mechanism investigation. (a) Photos of Zw-AuNPs with the addition of one or a mixture of M3+ in tap water, drinking water, or human urine. The final concentrations are listed on the top; (b) A520/650 value of S-AuNPs containing different metal ions with a concentration of 100 μM.

report, we also showed that the only ion that can aggregate AuNPs modified by positively charged quaternary ammoniumterminated thiols is Hg2+ as it is the only ion that can abstract the thiol off the surface of modified AuNPs to cause instability. As a control experiment, we modified the surface of 13 nm AuNPs with only negatively charged sulfonate-terminated thiols (Figure 5b). The zeta potential of sulfonate capped AuNPs (SAuNPs) is −31.4 ± 2.1 mV, and S-AuNPs are well-dispersed in aqueous solutions due to the strong electrostatic repulsion. SAuNPs showed response to 8 metal cations (including a few trivalent ions, but also divalent ions without discrimination) among the 18 metal ions (Figure 5b). Thus, the response of Zw-AuNPs to M3+ is neither solely attributed to the negatively charged sulfonate terminated thiols nor the positively charged quaternary ammonium terminated thiols. In the case of Zw-AuNPs, thiols can construct a zwitterionic surface on AuNPs. The stability of Zw-AuNPs is determined by both the van der Waals force and the electrical interaction. We measured the zeta potential of Zw-AuNPs before and after the addition of M3+. The addition of Fe3+, Al3+, or Cr3+ can change the zeta potential of Zw-AuNPs from −16.3 to almost zero, whereas such changes cannot be observed for other metal cations (Table 1) The phenomenon also coincides with the recently published research on the interactions between the zwitterionic micelles and metal cations,24 which concluded that trivalent cations can significantly decrease the negative zeta potential of zwitterion micelles. The variation in the zeta E

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Table 1. Zeta Potential Measurements for Zw-AuNPs with M3+ (25 μM) −16.3 ± 2.1 −3.18 ± 0.53 −4.94 ± 0.8 −1.54 ± 0.4 −15.7 ± 3.1

Mixtures include 14 kind monovalent cations and divalent cations, the concentration of each cations were 100 μM.

potential would break the balance that disperse Zw-AuNPs and further trigger the aggregation.

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CONCLUSION We explore a colorimetric assay for M3+ based on Zw-AuNPs. M3+ (such as Cr3+, Al3+, Fe3+) can induce the change of polarized dipole of the ligands and result in the alteration of zeta potential that could trigger the aggregation of Zw-AuNPs. Due to the zwitterionic surface, Zw-AuNPs show good stability, recyclability, and selectivity for M3+. The utility of our assay has been examined by performing M3+ detection in tap water, drinking water, and urine samples. Compared with most previous reports on M3+ detection, the instrument-free and cost-saving features particularly endow our assay ideal for uses in resource-limited areas. We believe that by combining with some lab-on-chip formats such as microfluidic chips, our assay will be useful to develop point-of-care tests.

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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.6b00501. Additional information as noted in text (PDF)

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zeta potential (mV) blank 25 μM Fe3+ 25 μM Al3+ 25 μM Cr3+ mixtures

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Corresponding Authors

*E-mail for X.J.: [email protected]. Fax: (+86)1082545631. Phone: (+86)10-82545558. *E-mail for J.J.: [email protected]. Fax: (+86)-571-87953729. Phone: (+86)-571-87953729. Present Address ⊥

(W.C.) Department of Biomedical Engineering, Medical school, Shenzhen University, Guangdong, 518020, P.R. China

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by National Natural Science Foundation of China (21222502, 91213305, 21575032, 51373043, 81361140345, 21574114), the Ministry of Science and Technology (2011CB933201), Youth Innovation Promotion Association (CAS), the CAS/SAFEA International Partnership Program for Creative Research Teams, the “Strategic Priority Research Program” of the Chinese Academy of Sciences, Grant No. XDA09030305, National High Technology Research and Development Program of China (2012AA022703), the Ministry of Health (2012ZX10001-008) and the CAS/SAFEA International Partnership Program for Creative Research Teams for financial support. F

DOI: 10.1021/acs.analchem.6b00501 Anal. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.analchem.6b00501 Anal. Chem. XXXX, XXX, XXX−XXX