Mixing-to-Answer Iodide Sensing with Commercial Chemicals

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Mixing-to-Answer Iodide Sensing with Commercial Chemicals Yuexiao Jia, Wenshu Zheng, Xiaohui Zhao, Jiangjiang Zhang, Wenwen Chen, and Xingyu Jiang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b02126 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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Analytical Chemistry

MixingMixing-toto-Answer Iodide Sensing with Commercial Chemicals Yuexiao Jia,†,‡ Wenshu Zheng,†,‡ Xiaohui Zhao,† Jiangjiang Zhang,†,‡ Wenwen Chen,† 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, Beijing 100190, China. ‡

University of Chinese Academy of Sciences, Beijing 100049, China.

ABSTRACT: We develop a convenient, colorimetric assay (Au/PEI) for rapid iodide (I-) determination that can be prepared facilely by mixing commercially available chemicals including tetrachloroauric acid (HAuCl4) and polyetherimide (PEI), and carried out directly by adding the samples to the assay without any pretreatment and additional procedure. Au/PEI operate on the principle that I- accelerate the formation of Au NPs which leads to a visible color change from light yellow to red for naked-eye readout with high specificity. We integrate our assay on solid devices including gel hybrids (Au/PEI/GH) and filter paper (Au/PEI paper) to satisfy the demand of point-of-care testing and justify the practicality by detecting I- in lake water that was supplemented with 10, 20, or 40 µM of I-. Au/PEI/GH with the limit of detection of 0.35 µM can satisfy the detection of drinking water based on the guidelines (1.2 µM) set by Chinese government, and Au/PEI paper can be used even after one-year storage. Such assays provide convenient and straightforward choice for routine, onsite I- tests.

Iodine is an essential trace element on earth and a micronutrient for human. It is necessary for normal human growth and thyroid function.1 A daily intake of 150 μg is necessary for adults and 250 µg during pregnancy and lactation. Disorders of iodine could lead to many diseases such as cretinism, goiter, hypothyroidism, and hyperthyroidism.2-3 The iodine standard in drinking water set by Chinese government is 150 μg/L (1.2 µM). Iodine in nature sources is unevenly distributed and is commonly associated with endemic thyroid dysfunctions.4 For example, groundwater iodine concentration level in the North China Plain varies between 1.51 μg/L and 1106 μg/L, and 32.3% of groundwater iodine is higher than the guidelines for drinking water.5 Iodine is also used in chemical synthesis which ends up in industrial wastewater, and iodometric method is applied in the detection of analysts such as sulfide hydrogen, glucose and organic peroxides.6-7 Sensing of iodide (I-) in surrounding environments and chemical samples is necessary due to its crucial roles in chemical and biological processes. Existing methods for detecting I- have several major limitations. The most generalized assays Sandell–Kolthoff reaction, chromatography 8, ICP mass spectrometry (ICPMS)9, and electrochemical method10 rely on complicated fabrication, expensive instruments, rigorous environments, professional operators or significant time consumption. In comparison, gold nanoparticles (Au NPs)based colorimetric methods that enable naked-eye readout have attracted much attention in the determina-

tion of various analytes, and show great promises for the development of portable devices for point-of-care testing (POCT).11 Due to the easy synthesis and diverse surface chemistry, various functionalized Au NPs have enabled the detection of metal ions 12-14, glucose15, peptide16, enzyme17 and pesticides18. For I- sensing, fluorescence turnon sensors are available based on ligand displacement of fluorescent molecules decorated Au NPs.19-22 I- can induce morphology change of Au NPs which is applied in the colorimetric assays for I-.23-24 Aggregation and antiaggregation methods are also carried out based on the distance dependent optical properties of Au NPs.25-27 However, these sensors commonly involve poor stability, complex pretreatments and external components which will hinder the convenience of the probe. Solid analytical device is a kind of convenient and easyoperate strategy with practicality in on-site testing for targets like H2O2 and H2S in environmental and biological samples.28 Au NPs based solid sensors are developed by loading functionalized Au NPs onto paper, membrane, glass and polymers to improve the stability and long-time storage of Au NPs based probes.29 For I- sensing, an Au NPs decorated mixed cellulose ester membrane (Au NPs/MCEM) is constructed.30 Although it realizes high sensitivity, it involves complex fabrication and multi-step operation and additional components. So it still remains a great challenge to develop convenient and straightforward Au NPs-based assays and solid devices for the determination of I-.

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Herein, we report a straightforward strategy for I- sensing that can be prepared and carried out simply (Scheme 1). The sensing system (Au/PEI) is a mixing-to-answer assay that can be prepared by a short-time mixing of commercially available tetrachloroauric acid (HAuCl4) and polyetherimide (PEI). PEI is a cationic polymer used as polyvalent ligand to modify and stabilize nanoparticles relying on the strong interaction of amine groups with metal atoms.31-32 It can facilitate the synthesis of Au NPs as both the reducing and stabilizing agent, but the reaction activity is weak at room temperature. In this report, we observe that I- can greatly accelerate the generation of Au NPs with high specificity to result in obvious color change and absorbance increase of Au/PEI. We employ this phenomenon to develop a straightforward method for I- sensing. After testing the detection performance in aqueous, solid sensing strategies including Au/PEI paper and Au/PEI gel hybrids (Au/PEI/GH) with more stable and easy-operated features in complex samples and during storage are fabricated for POCT. Au/PEI/GH achieves the limit of detection (LOD) of 0.35 µM satisfying the Idetermination in drinking water. Validity period of Au/PEI paper can be as long as one year which is promising in POCT. We apply these devices in the determination of I- in lake water supplemented with I-. Thus, our assay provides a convenient and straightforward strategy to monitor I- levels of chemical and environment samples. Scheme 1. Mixing-to-Answer Colorimetric Assay and Solid Devices for I Sensing Based on I Accelerated Formation of Au NPs.

EXPERIMENTAL SECTION Materials and Instruments. Polyetherimide (PEI, 25000 kDa), tetrachloroauric acid (HAuCl4, 99.99 %) and sodium iodide (NaI, 99.99 %) are from Sigma-Aldrich and used as received. The water used throughout all experiments is from a purified water system (Youpu UPT-II-10T, Beijing). Lake water is from Tsinghua University. We take transmission electron microscope (TEM) images using a Tecnai G2 20 S-TWIN. The UV-vis spectra are recorded using a UV2450 spectrophotometer (Shimadzu) and microplate reader (Infinite M200 PRO, TECAN). Optical photographs are recorded by a Nikon D90 camera. Preparation of Au/PEI and I- Sensing. We dissolve 100 mg PEI into 25 mL deionized water. Then we add 500 µL HAuCl4 to the solution (the final concentration is 0.8 mg/mL). The mixture is stirred for 5 min at 60 oC. To detect I-, we add 100 µL I- solutions to 100 µL Au/PEI and

incubate them for 60 min. We record the UV-vis spectra of the mixture and take pictures. Optimization of Preparing Condition of Au/PEI. To test the necessary of the components and the incubation, we add I- to HAuCl4, the mixture of HAuCl4 and PEI, and Au/PEI with final concentration of 20 µM. We also explore some other polymer including poly-lysine (PL), polyvinyl pyrrolidone (PVP) and polyethylene glycol (PEG) for the fabrication of I- sensors (Au/PL, Au/PVP and Au/PEG) following the same protocol as Au/PEI’s by incubating HAuCl4 with them at 60 oC for 5 min. To get the optimized sensing system, we prepare Au/PEI with different ratio of Au and PEI including 2.5 (Au/PEI2.5), 5 (Au/PEI5), 10 (Au/PEI10), 20 (Au/PEI20), by adding different amount of PEI (50, 100, 200 and 400 mg). We optimize the incubating time by incubating the mixture of HAuCl4 and PEI for different lengths of time at different temperatures (40, 50 and 60 oC). After incubating the mixture at different conditions, we add I- with the final concentration of 20 µM. We record the A530 increases of Au/PEI before and after the addition of I-. Optimization of Sensing Condition of Au/PEI. We investigate the time, temperature and pH effects on the sensing process of Au/PEI. We add I- (final concentration: 20 µM) to Au/PEI, and monitor the A530 increase at different time. We test the effect of temperature on the sensing process by incubating the sensing system at three different temperatures (25, 37, 60 oC). We also test pH effect on the sensing process by adjusting Au/PEI to different pH and adding I- solution to a final concentration of 20 µM. We record the A530 changes after adding I-. Selectivity and Sensitivity of Au/PEI. We test the sensitivity of the assay by adding different concentration of I- to the Au/PEI solution. The final concentrations of Iare set to be 0, 1, 2.5, 5, 7.5, 10, 12.5, 15, 17.5, 20, 22.5, 25, 30 and 50 µM. To evaluate the selectivity of the assay, we add different anions to Au/PEI. The anions include PO43-, HPO42-, H2PO4-, CO32-, HCO3-, SO42-, SO32-, S2-, NO3-, NO2-, F-, Cl-, Br-, OH-, Ac-, SCN-, IO3- and citrate (final concentration: 20 µM). After 60 min of incubation, we take pictures and record the UV-vis spectra. We test the response of Au/PEI to I2 by adding I2 solution to Au/PEI (final concentration is 20 µM). We also test the influence of high concentration (100 µM and 500 µM) of Br-, citrate, PO43-, SO42- and CO32- and the combination of these anions with I -. Fabricate Au/PEI Paper. Typically, we infiltrate the filter paper (diameter: 5 cm) with 1 mL of the light yellow Au/PEI, and freeze-dry it. We can obtain a white Au/PEI paper. We cut the paper into square with the side length of 8 mm. We next characterize the range of concentration of Au/PEI paper by dropping 100 µL of I- solution onto the test paper and incubate them for 1 h. Construct Au/PEI Gel Hybrids. We build Au/PEI gel hybrids by mixing the boiling agarose with HAuCl4 and PEI. Typically, we dissolve 200 mg PEI to 25 mL H2O and add 1 mL HAuCl4 (40 mg/mL) to PEI solution. We heat the agarose solution (0.25 %) with microwave oven for


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-

Figure 1. Enhancement effect of I on the formation of Au NPs. (a) Photographs of HAuCl4, Au/PEI in the absence and presence of I . (b) UV-vis spectra of HAuCl4, Au/PEI in the absence and presence of I . (c) Possible mechanism of I accelerated formation of Au NPs. (d) TEM images of Au/PEI in the absence and presence of I at 0.1 µM, 1 µM, 5 µM, 10 µM and 20 µM. The scale bar is 50 nm.

2 min. Then we mix the boiling agarose with HAuCl4 and PEI. The mixture is transferred to 96-well plates within 5 min and solidified at room temperature. To detect I-, we add I- solution into the 96-well plate and incubate the plate for 60 min. We record the UV-vis spectra by microplate reader. Analysis of I- in Lake Water. We add I- to lake water with final concentration of 10, 20 and 40 µM. To detect Iin lake water samples, we directly add 100 µL of the samples to the three devices, and incubate them for 60 min. Then we record the UV-vis spectra of the solutions and take photographs. We calculate the concentration of I- in lake water based on A530. We also detect the concentration of I- in lake water by an assay kit based on Sandell– Kolthoff reaction method and compare the detection results of these methods.

RESULTS AND DISCUSSION DISCUSSION Iodide Accelerated Au NPs Formation. We prepare Au/PEI by directly incubating aqueous HAuCl4 and PEI for 5 min. PEI is commonly used to synthesize Au NPs, but the reaction is temperate and Au/PEI remains light yellow even after 24 h. While in the presence of I-, the color of Au/PEI gradually changes from light yellow to red in 1 h (Figure 1a). We monitor the preparing and sensing processes of Au/PEI by following the UV-vis spectrum (Figure 1b). Aqueous HAuCl4 shows an absorption peak at 306 nm (A306). In Au/PEI, it decreases obviously with a red shift due to the coordination and reduction of Au3+ with amines of PEI.33 During I- sensing, Au/PEI shows absorption increase with the appearance of an absorption peak at 530 nm (A530). The color change and absorbance increase indicate the generation of colloidal Au NPs. Transmission electron microscopy (TEM) study is applied to give insight into the formation of Au NPs (Figure 1d). We cannot observe Au NPs in the TEM image of Au/PEI. With the addition of small amount of I- (0.1 µM), there

emerge many small Au NPs, and the sizes of Au NPs increase gradually with raising [I-]. When [I-] is 5 µM, Au NPs in the size of about 20 nm are observed which contribute to the UV-vis absorption peak at 530 nm and increase when more I- (10 µM and 20 µM) are added. The synthesis of Au NPs commonly consists of two steps: nucleation and particle growth. Au ions are reduced to form nuclei and the nuclei undergo coalescence and further grow to Au NPs.34 Stabilizing reagents like polymers can provide electrostatic or steric stabilization to control the growth of Au NPs.35 In this report, PEI act as both reducing and stabilizing reagent. The amines of PEI can reduce Au3+ to Au0 to produce nuclei. At the same time, they will adsorb to the surface of as-formed nuclei producing electrostatic and steric barrier, reducing the coalescence chance of nuclei and slowing the further growth of Au NPs. Iodide has stronger affinity with Au NPs and can replace the amines on the surface of nuclei to reduce the electrostatic and steric barrier between nuclei, increase their chance of coalescence and accelerate their growth to Au NPs (Figure 1c).19-20 Au/PEI recognizes I- based on I-accelerated formation of gold nanoparticles. We apply this phenomenon in a colorimetric assay for rapid I- determination. Optimization of Au/PEI and Sensing Condition. We optimize the parameters of preparation processes to improve the convenience and sensitivity of Au/PEI. We test the necessary of the components and incubation by adding I- to HAuCl4, the mixture of HAuCl4 and PEI, and Au/PEI solution. I- cannot cause the same absorbance increase and color change for neither HAuCl4 nor the mixture of HAuCl4 and PEI. So the addition of PEI and incubation are necessary for the preparation of Au/PEI (Figure 2a, S1). We have explored polymers other than PEI including poly-lysine (PL), polyvinyl pyrrolidone (PVP) and polyethylene glycol (PEG) for this process. These polymers don’t have the same effect as PEI and cannot be



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experiences the most obvious response to I- (Figure 2c, S4). We also evaluate the time, temperature and pH effects on the sensing process to find a suitable detecting condition by carrying out the sensing process in different conditions. The A530 increases obviously within 15 min, and the reaction is accomplished after 60 min. So we choose 60 min as the incubating time (Figure 2d). Temperature change can increase the absorption slightly. For convenience, we carry out the sensing process at room temperature (Figure 2e). We finally evaluate the pH influence on the response of Au/PEI to I- by adjusting pH values of Au/PEI from 4 to 12. The result shows that our system can work on the pH range of 7 to 9 (Figure 2f).

Figure 2. Optimization of preparation and sensing conditions. (a) Responses of HAuCl4, the mixture of HAuCl4 and PEI, and Au/PEI to I . (b) Optimization of weight ratio of PEI and Au. (c) Optimization of incubating temperature and time of preparation of Au/PEI. Effects of (d) the incubating time, (e) incubating temperature and (f) pH on I sensing.

used to fabricate sensors for iodide (Figure S2). PEI can be used as both reducing and stabilizing reagent and it is essential in the formation of Au NPs and iodide sensing. We optimize the amount of PEI by preparing Au/PEI with different weight ratio of PEI and Au (2.5, 5, 10 and 20). When the ratio is 5, the system experiences the most obvious color and absorbance changes with the addition of I(Figure 2b, S3). We also optimize the incubation time and temperature. Au/PEI is prepared in water bath at 40 oC, 50 oC and 60 oC. When incubated at 60 oC for 5 min, it

Selectivity and Sensitivity. We test the sensitivity and selectivity of Au/PEI by color change and absorption increase. We evaluate the sensitivity of Au/PEI by adding different concentration of I-. Au/PEI performs a gradual color change from light yellow to red with increasing [I-]. There is a visible color change when [I-] is 5 µM (Figure 3a). There is an absorption peak at 530 nm (A530) in UVvis spectra which increases obviously with rising [I-] (0-50 µM, Figure 3b). The reaction tends to be saturated when [I-] is 25 µM. The concentration dependent absorption curve shows a linear relationship in the range of 5 to 25 μM of I- (R2=0.9887) and the LOD is 3.7 µM (Figure 3c). To evaluate the specificity, we test the responses of Au/PEI to different anions (PO43-, HPO42-, H2PO4-, CO32-, HCO3-, SO42-, SO32-, S2-, NO3-, NO2-, F-, Cl-, Br-, OH-, Ac-, SCN-, IO3-, and I-) with the final concentration of 20 µM. The A530 of Au/PEI in presence of I- is much higher than that for a panel of other anions (Figure 3e). The color of Au/PEI changes from light yellow to red with the addition of I-. While after incubation with other anions, the solution remains light yellow without visible color change (Figure 3d). I- is an anion with reductive property which is commonly used to be recognized among other anions. So we test the interference of a reductant (citrate) for iodide.

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Figure 3 Sensitivity and selectivity of Au/PEI. (a) Photographs of Au/PEI in the presence of different concentrations of I (0-25 µM). (b) UV-vis absorption spectra of Au/PEI with the addition of I (0-30 µM). (c) A dose–response curve and linear relationship of A530 to [I ] (0-25 µM). (d) Photographs of Au/PEI in response to various anions and citrate. (e) A530 value of Au/PEI with the addition of different anions and citrate (20 µM).


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Analytical Chemistry Citrate does not cause color change and absorption increase. Iodine (I2) can also induce color change and absorption increase of Au/PEI. It is reported that iodide and iodate rather than iodine are the commonly existing inorganic species in groundwater and iodide is the dominant species in environmental samples. The concentration of iodide can reveal the iodine level in the area. So we mainly evaluate the determination of I- by Au/PEI.5 To further demonstrate the selectivity of Au/PEI, we evaluate the response of Au/PEI to some anions at high concentration (100 µM and 500 µM) like Br-, citrate, PO43-, SO42- and CO32-, and the mixture of I- (20 µM) with them. These anions themselves cannot cause color change of Au/PEI even at 500 µM (Figure S5a, b). With the addition of the mixture of I- and these anions, the color of Au/PEI changes gradually from light yellow to red, purple, blue or black (Figure S5c, d). The fact that Au/PEI changes to purple, blue or black rather than red in the presence of the mixture of I- with high concentration (500 µM) of citrate, PO43-, SO42- and CO32- is because of the aggregation of Au NPs. Only I- enhances the generation of colloidal Au NPs. Thus our system has perfect selectivity which can be applied in I- sensing. As for the aggregation of Au NPs, the following introduced solid device (Au/PEI/GH) can solve the problem efficiently protecting Au NPs from aggregation with stable color changes from light yellow to red even in the presence of 500 µM of these anions (Figure S5e, f). Solid Devices. Following the above results, we try to construct solid devices based on Au/PEI. Although liquid sensors are usually able to provide quantitative detections for the analytes, they always suffer from complicated pretreatment and poor stability due to the effects of interferences and storage conditions. Solid devices are more convenient and stable under complex sensing conditions and during long-time storage which are requisite in point-ofcare detections. Superior to the aggregation and antiaggregation based method, Au/PEI is suitable to be

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Figure 4 Solid devices for I sensing. (a) Photographs of Au/PEI/GH with different concentrations of I . (b) UV-vis absorption spectra of Au/PEI/GH with I (0-100 µM). (c) Dose–response curve and linear relationship of A530 to [I ] (050 µM). (d) Photographs of Au/PEI paper in response to different concentrations of I (0-50 µM).

applied on solid substrates benefiting from the in situ generation of gold nanoparticles. We build Au/PEI gel hybrids (Au/PEI/GH) by mixing HAuCl4, PEI and boiling agarose. The mixture is transferred to 96-well plate and solidified at room temperature. Consistent with Au/PEI, Au/PEI/GH experiences a color change from light yellow to red in a concentrationdependent manner. There are separate color layers in the solid phase where the red layer thickens with rising [I-]. The red color in Au/PEI/GH results from diffusion of Ifrom the solution and in-situ generation of AuNPs.36-37 Diffusion rate is related to the concentration gradient between solution and gel hybrids leading to length variation of the red layer. There is a visible color change when [I-] is 2.5 µM (Figure 4a). Furthermore, the A530 increases with increasing [I-] and there is a linear relationship of A530 with [I-] ranging from 0 µM to 50μM with a LOD of 0.35 µM (Figure 4b, c). This LOD satisfy the requirement for POCT detection of drinking water. In addition, this POCT device can be used just as thin layer chromatography to monitor the reaction process during chemical synthesis. We also develop length-based devices based on Au/PEI/GH. Au/PEI/GH tips and tubes are fabricated by encapsulating Au/PEI/GH into pipettes and micro-pipette tips. After adding I- for 4 h, they show clear color changes where the length of red increase with rising [I-]. We take Au/PEI/GH tube as an example to study the lengthdependent performance of the device and find that there is a linear relationship between Log10[I-] and the length of Au/PEI/GH tube which can be used to detect I- semiquantitatively (Figure S6). The test tubes and tips provide simple and convenient devices for I- determination. They can achieve qualitative and semi-quantitative detection of I-, if one only relies on the length of the red layer. Aside from gel hybrids, we fabricate I- test paper based on a freeze-drying procedure using filter paper. Test paper is another convenient solid device usually applied for qualitative detection of analytes like H2S and H2O2. Filter paper is a hydrophilic material we use here to store Au/PEI and as a supporting substrate to protect Au/PEI NPs from aggregation. We freeze-dry the filter paper infiltrated with Au/PEI solution, and obtain a white Au/PEI paper. We characterize the range of [I-] by dropping 100 µL I- solution onto the Au/PEI paper. There is a gradual color change from white to red in response to I- in the range of 0-50 µM. The color change is visible in the presence of I- at 5 µM (Figure 4d). So the Au/PEI paper can be a convenient device for qualitative I- sensing. I- Sensing in Lake Water. To confirm the practical application of the three devices for I- analysis, we add lake water samples to Au/PEI, Au/PEI/GH and Au/PEI paper directly. Lake water without the supplement of iodide cannot cause color change and A530 increase of the devices. Detection results show that the concentration of iodide in lake water is below the LOD of the devices (0.08 µM). After the addition of samples with the supplement of 10, 20 or 40 µM of I-, Au/PEI shows obvious color changes



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Figure 5 I sensing in lake water. Photographs and UV-vis absorption spectra of (a) Au/PEI (b) Au/PEI/GH in response to lake water containing different concentrations of I (0, 10, 20, 40 µM). (c) Detection results of I in lake water by Au/PEI and Au/PEI/GH and Sandell-Kolthoff reaction method. (d) Photographs of Au/PEI paper stored for different length of time in the presence of lake water.

from light yellow to pink and blue in response to the lake water (Figure 5a). The color changes to blue instead of red with high concentration of I- (20 µM and 40 µM) indicating that the Au NPs aggregate due to the interferences. UV-vis spectra also demonstrate the aggregation of Au NPs with absorbance increase at high wavelength from 600 nm to 900 nm. Based on A530, Au/PEI can successfully detect I- at low concentration (recovery of 105.9 % for 10 µM). However, Au/PEI cannot give accurate results for high concentrations of I- (recovery of 86.9 % and 79.6 % for 20 µM and 40 µM respectively, Figure 5c). Lake water contains many anions like PO43-, SO42- and CO32- that will induce the aggregation of positively charged Au NPs.38 So we think these anions are the possible interferences in lake water.

Solid devices can solve the problem effectively because they provide supporting substrates to protect NPs from aggregation. After the addition of samples, Au/PEI/GH has obvious color changes from light yellow to red, and the color changes are stable during incubation (Figure 5b). Au/PEI/GH can detect I- in lake water accurately reaching detecting recoveries from 94.1 % to 106.4 %. The detection results obtained by Au/PEI/GH are consistent with that by Sandell–Kolthoff reaction method,1 a standard method for I- determination in urine, salt and water samples, which indicate that Au/PEI/GH can be a quantitative device for point-of-care testing of iodide. In addition, Au/PEI paper can also provide obvious color changes in response to the samples even after one-year storage which is useful for on-site qualitative sensing (Figure 5d). Thus, our method provides convenient devices for I- determination in environmental samples.

To improve the applicability of liquid probes, complex pretreatments and external components are always introduced which will hinder the convenience of the probe. Table 1 A Comparison of AuNPs-based Assays for I- Determination. Methods

Mechanism

Steps/Time

Chemicals

LOD

Solid devices

Thymine-AuNPs ssDNA-Au NPs

Anti-aggregation

5 /16 h

9

10 nM

No

25

Anti-aggregation

6 /2 h

6

13 nM

No

27

FITC−BSA−Au

Ligand displacement

5/10 h

8

50 nM

No

22

PolyA-tailed and fluorophore labeled aptamer-AuNPs

Ligand displacement

9/42 h

10

89 pM

No

21

Cu@Au

Morphology change

5/25 h

4

5 µM

No

24

Au NPs/MCEM

Anti-leaching

5/3 h

8

2 nM

Yes

30

Au/PEI

Growth of Au NPs

2/65 min

2

0.35 µM

Yes

This method

Compared with other AuNPs-based assays, Au/PEI is most convenient for I- determination (Table 1). Other AuNPs-based methods identify I- based on antiaggregation of Au NPs, I- induced ligand displacement and morphology change of Au NPs. They usually need

Reference

many operation steps (5-9) and chemicals (4-10) for synthesis, modification and purification of Au NPs and to induce their aggregation or leaching which result in complex operation, high cost and long consumption of time (2h-42 h). In contrast, Au/PEI is a mixing-to-answer assay


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Analytical Chemistry that can be used only after a short-time mixing (5 min) of two commercial chemicals and only involves two steps from preparation to sensing. In addition, Au/PEI can be integrated on solid devices (Au/PEI/GH and Au/PEI paper) which are more stable for complex samples and during long-time storage. Compared with the only one reported AuNPs-based solid device (Au NPs/MCEM), our assays are much more convenient both in fabrication and sensing which are promising in the POCT application. Combined with other technologies, such as surface-enhanced Raman scattering, this approach might improve sensitivity.

CONCLUSION We develop a straightforward colorimetric assay (Au/PEI) for I- determination based on iodide-accelerated formation of Au NPs. Au/PEI can be prepared facilely by a short-time (5 min) mixing of commercially available chemicals and carried out directly by adding samples to the assay without any sample pretreatment and external component. Solid devices including Au/PEI/GH (LOD is 0.35 µM) and Au/PEI paper are fabricated to facilitate convenient use and storage which are valid in lake water analysis that was supplemented with 10, 20, or 40 µM of I-. Our assay shows great promise for on-site I- testing and provide a general strategy for Au NPs-based solid device fabrication.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional information as noted in text (PDF)

AUTHOR INFORMATION Corresponding Author *Tel.: (+86)10-82545558. Fax: (+86)10-82545631. E- mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank the Ministry of Science and Technology of China (2013YQ190467), Chinese Academy of Sciences (XDA09030305) and the National Science Foundation of China (81361140345, 21535001, 81730051) for financial support.

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