Disposable Paper-based Analytical Device for Visual Speciation

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Disposable Paper-based Analytical Device for Visual Speciation Analysis of Ag (I) and AgNPs in Commercial Products Qinlei Liu, Yao Lin, Jing Xiong, Li Wu, Xiandeng Hou, Kailai Xu, and Chengbin Zheng Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b04609 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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Analytical Chemistry

Disposable Paper-based Analytical Device for Visual Speciation Analysis of Ag (I) and AgNPs in Commercial Products Qinlei Liu,† Yao Lin,† Jing Xiong,† Li Wu,‡ Xiandeng Hou,†,‡ Kailai Xu,†* and Chengbin Zheng†* †

Key Laboratory of Green Chemistry & Technology of MOE, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China ‡ Analytical & Testing Center, Sichuan University, Chengdu, Sichuan 610064, China ABSTRACT: A disposable, instrument-free, height readout paper-based analytical device (HR-PAD) based on a paper strip inkjet-printed with CdTe QDs was developed for the sensitive speciation analysis of Ag+ and AgNPs in commercial products. When the paper strip is immersed into a sample solution, capillary action draws it over the surface and any Ag+ in the solution quenches the fluorescence of CdTe QDs via a cation exchange reaction between Ag+ and the CdTe QDs, with the height of the quenched band being proportional to the concentration of Ag +. On the contrary, fluorescence quenching cannot be observed when only AgNPs are present in solution. Thus, the concentration of AgNPs can be obtained by subtracting the Ag+ content from the total silver determined by the HR-PAD after digestion with HNO3. Under optimized conditions, the methodology provides high selectivity, sensitivity and accuracy for the detection of Ag+ or AgNPs in various samples, even at concentrations as low as 0.05 mg L -1. Precisions of 4.5% and 2.2% RSDs were achieved at concentrations of 1 mg L-1 and 7 mg L-1 of Ag+, respectively. Compared to conventional methods, this approach is inexpensive, user-friendly and eliminates the need for expensive and sophisticated detection instruments. The practicality of the method was demonstrated via the speciation analysis of AgNPs and Ag+ in river water and twelve commercial products with satisfactory results.

Due to their unique antimicrobial, optical, and electrochemical properties, silver nanoparticles (AgNPs) have been widely used in consumer products over the past decade.1,2 As a result of the usage and disposal of these commercial products, AgNPs will inevitably be released into the environment and may pose a potential threat to the human and eco-environmental system.3-5 Although the risk associated with the use of AgNPs has attracted significant attention, the exact mechanism for AgNPs toxicity is still unclear. In general, it is suggested that most of the adverse effects are caused by the Ag+ released from AgNPs because Ag+ can produce reactive oxygen species (ROS) to trigger oxidative stress in organisms.6-8 However, some previous studies9-11 also reported that AgNPs could easily pass through cell membranes and contribute to anadditional toxicity mechanism by mutating DNA and disrupting cellular metabolism. Therefore, the identification and speciation analysis of AgNPs and Ag+ is crucial, not only to protect human from exposure to AgNPs, but to also understand and elucidate the environmental transport, transformation, and toxicity properties of AgNPs.

Ag. Although these techniques provide high sensitivity for the speciation analysis of AgNPs and Ag+, they are tedious and suffer from low throughput. Single particle inductively coupled plasma mass spectrometry (SP-ICP-MS) and surfaceenhanced Raman spectrometry (SERS) are techniques having excellent sensitivity and high throughput for the identification of AgNPs,14,15 whereas the direct speciation analysis of Ag+ and AgNPs by these techniques has not yet been reported. In contrast to non-hyphenated techniques, the hyphenated techniques (chromatography, field flow fractionation or capillary electrophoresis coupling to inductively coupled plasma mass spectrometry, ICP-MS) can directly accomplish the speciation analysis of AgNPs and Ag+.16,17 However, they still require expensive instrument and sophisticated operator, thus hindering their field analysis or identification of AgNPs at home. Thus, it is attractive to develop inexpensive and equipment-free approaches for ordinary families to identify and selectively quantify AgNPs or Ag+ in commercial products. Recently, microfluidic paper-based analytical devices (µPADs) have been widely applied for analysis of metal ions because they are extremely low-cost, disposable and simple.18-22 Importantly, these µPADs can be directly and in situ operated in place where technical infrastructure is limited and trained experts are absent, such as developing countries, field analysis, or private homes. In general, determination of metal ions or biomolecules with µPADs is obtained based on variation in colour intensity,23-26 however, their accuracies are susceptible to variation in the user’s interpretation (especially with colour-blind or colour-weak individuals). Thus, most of these methodswere only used for qualitative

Great efforts have been dedicated to develop reliable techniques for the speciation analysis of AgNPs and Ag+, which can be simply divided into non-hyphenated and hyphenated approaches.12-15 For non-hyphenated techniques, total amounts of Ag are often measured by atomic spectrometry after digestion, while Ag+ is selectively determined using anion-selective electrode or atomic spectrometry after separation of Ag+ from AgNPs.12,13 Consequently, the concentration of AgNPs can be obtained by subtracting the amount of Ag+ from the total content of

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and semiquantitative analysis. To overcome these drawbacks, distance-readout microfluidic quantitative detection methods have been developed for disease diagnosis in home healthcare, accident sites and emergency situations.27-32 With these approaches, the targeted analytes initiate a specific reaction to generate a colored band on a substrate or produce gas to expand visualized ink to a certain distance.27,31,32 These distances are proportional to the target concentrations and can be observed with the naked eye. Compared to intensity-based methods, these approaches not only retain the advantages of the conventional µPADs but also provide accurate result for the amount of analyte.To the best of our knowledge, there is no such distance-based method available for the speciation analysis of metal nanoparticles and their soluble ions.

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Reagents and Materials. All reagents used in this work were of at least analytical grade. Na2TeO3 and 3mercaptopropionic acid (MPA) purchased from Aladdin Reagent Co. (Shanghai, China) were used to prepare CdTe QDs. High-purity CH3COOH, HNO3, NaOH, Na2CO3, AgNO3, CdCl2, KBH4, Na3C6H5O7·2H2O and ethanol were bought from Kelong Reagent Factory (Chengdu, China). Stock solutions (1000 mg L−1) of Pb2+, Na+, Ca2+, Cr3+,Zn2+, Ni2+, Li+, K+, Bi3+, Mg2+, Fe3+, Cu2+, Hg2+, Hg+, Cd2+ and Ag+ were obtained from the National Research Center for Standard Materials (NRCSM) of China. The 20 mg L-1 AgNPs with different particle sizes (10, 40, 60, 80 and 100 nm) were purchased from Sigma Reagent Co. (Shanghai, China). Highpurity 18.2 MΩ-cm deionized water (DIW) obtained from a Milli-Q water system (Chengdu Ultrapure Technology Co., Ltd., Chengdu, China) was used throughout the experiments.

Herein, a height readout paper-based analytical device (HRPAD) was developed for the identification and speciation analysis of Ag+ and AgNPs. In this approach, the CdTe QDs inkjet-printed onto a paper strip inserted into a sample solution containing Ag+ results in the capillary movement of the Ag+ up the strip where it quenches fluorescence from the CdTe QDs due to a cation exchange reaction (CER) between Ag+ and CdTe QDs. The visual height of the quenched fluorescence band is proportional to the concentration of Ag+. Interestingly, no fluorescence quenching occurs due to the presence of AgNPs, even when present as small as 10 nm. Consequently, the approach was successfully used for speciation analysis of Ag+ and AgNPs in various samples.

Sample Collection and Preparation. Four river water samples were collected from two local rivers in Chengdu city and immediately analyzed after filtration through a 0.22 μm filter membrane. Twelve commercial products containing AgNPs were bought from local supermarkets and pharmacies, including textiles, gynecological lotions, surgical dressing and baby products. Owing to their different forms, the methods used for the preparation of these samples varied. For the textile, surgical dressing and baby products (tooth brush or rubber nipple), nitric acid-washed ceramic scissors were used to cut these samples into tiny fragments (about 0.001 cm2). Subsequently, 4 g subsamples of each sample were accurately weighed into precleaned glass beakers, and 20 mL DIW were added. The beakers were then placed in a water bath and exposed to an ultrasonicationat 20 W for 30 min. Finally, these mixtures were filtrated and the collected filtrates were analyzed as soon as possible. For the gynecological lotions, 5 ml subsamples were pipetted into precleaned 30 mL plastic bottles and diluted to 10 mL with DIW prior their direct determination by HR-PAD.

EXPERIMENTAL SECTION Instrumentation. A commercial F-7000 spectrofluorometer (Hitachi, Japan) equipped with a solid sensing cell was used to measure the fluorescence of the CdTe QD inkjet-printed paper strip. High-resolution transmission electron microscopy (HR-TEM) images were obtained with a Tecnai G2F20 S-TWIN transmission electron microscope at an accelerating voltage of 200 kV (FEI Co., U.S.A.). Scanning electron microscopy (SEM) was performed with a JSM-7500F scanning electron microscope (JEOL, Tokyo, Japan). An Xray diffractometer (X’Pert Pro MPD, Philips, Netherlands) using Cu Kα radiation was used to record the powder X-ray diffraction (PXRD) patterns. X-ray photoelectron spectroscopy (XPS, XSA M800, Kratos, UK) was also used to characterize the CdTe QD inkjet-printed paper strip before and after the reaction. A gel image analytical system equipped with a 365 nm reflected UV source (Beijing Junyi Instrument Factory, China) was used to directly observe the fluorescence quenching of the paper strip. An Agilent 1200 series liquid chromatography system coupling to an inductively coupled plasma mass spectrometer (ICP-MS, Agilent 7700cs) were used for the speciation analysis of AgNPs and Ag (I) in the tested samples. The operation conditions of LC-ICP-MS were summarized in Section 1 of the Supporting Information (SI). A pH meter (PHS-25, Shanghai, China) was used to permit adjustment ofthe pH of all solutions. An inkjet printer (EPSON M105) was used to print CdTe QDs onto chromatography paper.

Synthesis of CdTe QDs and Preparation of CdTe QDs Inkjet-Printed Paper. The synthesis of CdTe QDs was accomplished according to a reported method33 and briefly described in Section 2 of the SI. A novel method was developed to prepare the CdTe QDs inkjet-printed paper. Initially, the prepared solution containing 100 mg L-1 of CdTe QDs was poured into an empty cartridge of an inkjet printer (M105, EPSON, Japan). Thereafter, chromatography paper (12 × 13 cm, Whatman 3030-861, U.K.) was put into the printer and printed three times. Finally, the CdTe QDs ink-printed paper was cut into 2 mm x 13 cm paper sheets using a cutting machine (Deli, Ningbo, China) and then sealed in a plastic bag and kept at room temperature prior to use. Procedure for Speciation Analysis of Ag+ and AgNPs. Scheme 1 illustrates the procedure forspeciation analysis of Ag+ and AgNPs in water samples using the HR-PAD array. First, twenty-four CdTe QDs inkjet-printed paper strips were fixed on acrylic plates to form an HR-PAD array prior to

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analysis. Standard solutions or 1 mL sample solution were placed into a custom-made 24-well plate, each having a volume of 2.5 mL. For the determination of Ag+, the acrylic plates were directly inserted into the wells for 20 min at room temperature. Owing to capillary forces, Ag+ standard solution or sample solution automatically moved up the paper strip and reacted with the CdTe QDs. Finally, the acrylic plates were removed and transferred to a UV test box to accomplish visual height-readout for quantitative detection of Ag+ under 365 nm UV irradiation. For the detection of total Ag, 50 μL of concentrated HNO3 (68%, m/v) was added to the wells to efficiently dissolve the extracted AgNPs and then the pH of the digested solution was adjusted to 7.0 with a 1 M NaOH solution prior to the immersion of HR-PAD array. Consequently, the AgNPs content of the samples was obtained by subtracting the Ag+ concentration from the total Ag content.

microscopy coupled to energy dispersive spectroscopy (SEMEDS).

Figure 1. Feasibility of HR-PAD on quantification of Ag+. The colour change of the paper strip under room light (a) and under 365 nm UV irradiation (b), respectively; the height of fluorescence band quenching induced by various concentrations of Ag+ (c); and a calibration curve obtained for the detection of Ag+ (d).

The SEM images (Figure 2a, b and c) show the cellulosic fiber structure can be obviously visible in all the tested paper strips. Compared to the surface of the raw paper strip in absence of QDs, the surface of the CdTe QDs inkjet-printed paper strip becomes smoother, indicating that QDs have been evenly printed on the paper strip. After immersion into the Ag+ standard solution, the surface morphology of the CdTe QDs inkjet-printed paper strip was obviously changed, implying that a reaction between QDs and Ag+ occurred on the paper surface. Figures 2d, e and f display the EDS spectra collected from these tested paper strips. Apart from the characteristic signals belonged to Au, C and O can be also observed in the spectrum of the raw paper strip, additional peaks of Cd and Te are simultaneously detected in the spectra obtained from the CdTe QDs inkjet-printed paper strip before and after immersion to the Ag+ solution. In the spectrum of the QDs inkjet-printed paper strip after the immersion, obvious peak of Ag at 2.894 keV can be found, whereas the Cd peak at 3.316 keV is almost disappeared. These results not only suggest that the CdTe QDs can be easily printed on the paper strip but also confirm that the CER reaction occurs between CdTe QDs and Ag+.

Scheme 1. Theprinciple of speciation analysis of Ag+ and total Ag using HR-PAD.

RESULTS AND DISCUSSION Feasibility of HR-PAD on Quantification of Ag+ and Its Mechanism. Since there is no distance-based method available for the quantification of Ag+, two initial experiments were conducted to prove the feasibility of the HR-PAD for this purpose. First, two CdTe QDs inkjet-printed paper strips were immersed in 1 mL of blank solution and a standard solution containing 1 mg L-1 Ag+ for 30 min under ambient conditions. Compared to the paper strip immersed into the blank solution, about a 1.6 cm length of a light brown band (Figure 1a) was observed on the strip immersed in the standard solution. As can be seen from Figure 1b, the fluorescence generated from this band was completely quenched. A series of standard solutions containing various concentrations of Ag+ was analyzed to further investigate the feasibility of this method for the determination of Ag+. Figure 1c shows that the height of the fluorescence quenching is gradually increased with increasing concentration of Ag+. As shown in Figure 1d, a typical calibration curve with a good linear coefficient of 0.99 is obtained for the determination of Ag+ by the HR-PAD. All aforementioned analytical results demonstrate the feasibility of the HR-PAD for quantification of Ag+. In order to gain insight into the mechanism of fluorescence quenching, both a raw paper strip and a CdTe QDs inkjetprinted paper strip before and after immersion into Ag+ standard solution was characterized with scanning electron

In order to further confirm the CER reaction, the paper strip after immersion into Ag+ standard solution was placed into a quartz tube containing DIW and exposed to an ultrasonication at 20 W for 30 min. Finally, the extracted products was collected and characterized by TEM with the initial CdTe QDs. The TEM images (Figures S1a and S1b) show that the lattice spacing of the product is 0.28 nm, whereas that of the initial CdTe QDs is 0.24 nm, thus supporting the CER

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fluorescence band quenching obtained from 1 mg L-1Ag+ solution and a 1 mg L-1 80 nm AgNPs solution before and after dissolution in HNO3 was undertaken, as shown in Figure S8a. The results show that no obvious fluorescence quenching of the paper strip immersed in the solution of AgNPs is observed, implying that AgNPs cannot induce the CER. On the other hand, the heights of fluorescence band quenching resulting from exposure to the Ag+ solution and the dissolved AgNPs solution are almost the same, demonstrating that the HR-PAD provides the same sensitivity for the determination of both Ag+ and the dissolved AgNPs. Previous studies24,35 found that the release of Ag+ from AgNPs was accelerated with decreasing size of the AgNPs. Therefore, a further experiment was conducted to investigate the effect of the size of AgNPs on the height of fluorescence band quenching using 1 mg L-1 sample of AgNPs of various sizes (10, 40, 60, 80 and 100 nm). As can be seen from Figure S8b, the height of fluorescence band quenching obtained from all the tested AgNPs is not detectable, indicating that AgNPs cannot react with the CdTe QDs inkjet-printed on the paper strip even when their sizes are as small as 10 nm. These results demonstrate that Ag+ can be selectively determined, and the determination of total concentration of silver can be obtained after the dissolution of the sample with HNO3, thus establishing a disposable, equipment-free HR-PAD for the speciation analysis of Ag+ and AgNPs.

reaction occurred on the surface of the CdTe QDs. Most interestingly, the shapes of the initial and the product are nearly identical, which agree with those reported in previous study34. This previous study considered that the number of anions per nanocrystal is invariant after the cation exchange reaction, thus resulting in that the basic shapes of the initial nanocrystals are thus retained. However, the nature of CER is very complex and further studies are required to clearly insight its mechanism. The XPS and XRD Characterizations of the product have also been done to understand The exact product of this reaction. Figure S1c shows that XRD peaks of Ag2Te are present in the paper strip after exposure to Ag+. Significantly, the results summarized in Figure S2a and b show the XPS signature of the Ag 3d doublet for the resulting silver ion could clearly be observed after the CER reaction. These observations confirm that the CdTe QDs printed on paper were proportionally converted to Ag2Te via a room temperature cation exchange reaction between the CdTe QDs and Ag+, resulting in a proportional height of fluorescence quenching.

Interferences. According to previous studies,35,36 some coexisting ions such as Cu2+, Zn2+ and Pb2+ can also induce CERs in CdTe QDs and then influence the CERs induced by Ag+. In this study, effects of 17 coexisting ions including alkali metals, alkaline earth metals, and transition metal cations as well as some anions on the HR-PAD for the determination of Ag+ were investigated. Standard solutions containing 1 mg L-1 Ag+ were used with addition of different amounts of the tested ions, respectively. The analytical results showed that there were no obvious interferences from these tested ions except Cd2+, Cu2+ and Fe3+ (Table S2), even at concentrations 100-fold of Ag+. For Cd2+, Cu2+and Fe3+, their effect on the height of fluorescence band quenching was not significant when their concentration was decreased to 50-fold that of Ag+. These results agree well with those reported in previous studies,37,38 which suggested the activation barriers of CERs occurred between CdTe QDs and the tested ions are much higher than that required for the CERs induced by Ag+, thus alleviating their interferences on the determination of Ag+. Density Functional Theory (DFT) (See Section 7 of the SI) was applied to further study the capability of anti-interference from the proposed method by comparing the adsorption energies (Eads) calculated from Ag+ and other metal ions adsorbed on CdTe QDs, respectively. Considering the reliability and efficiency of the calculation, (CdTe)33 cluster was selected to model CdTe QDs. The Eads is usually defined as follows:

Figure 2. Identiﬁcations of CdTe QDs inkjet-printed on papaer and the reaction between the QDs and Ag(I). The SEM images of raw paper strip (a) and the paper printed QDs before (b) and after (c) immersing into Ag+ solution; the EDS spectrums of raw paper strip (d) and the paper printed QDs before (e) and after (f) immersing into Ag+ solution.

Experimental Conditions for the Detection of Ag+. The eﬀects of the QDs size, the reaction time, the pH and volume of reaction solution on the height of the fluorescence quenching band were carefully investigated using 1.0 mg L−1 Ag+ standard solution, with results summarized in Section 4 of the SI. Feasibility of Speciation Analysis of Ag+ and AgNPs. Two experiments were undertaken to demonstrate the feasibility of the proposed method for speciation analysis of AgNP sand Ag+. A comparison between the heights of

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Analytical Chemistry Eads=Eadsorbate/cluster –Ecluster– Eadsorbate

Analytical Performance. The performance of the proposed method was evaluated under optimal conditions by using a series of standards containing various concentration of Ag+. As shown in Figure 4a, the height of fluorescence band quenching increased with increasing Ag+ concentration over the range of 0−30 mg L-1, followed by stable response at higher concentrations. The proposed method is very sensitive for the accurate determination of Ag+, even when its concentration is as low as 0.05 mg L-1, which completely meets the requirement of the Canadian Health Act Safe Drinking Water Regulation and the Standards for Drinking Water Quality of China for routine analysis of silver in water. A typical calibration curve with a linear coefficient of 0.999 and a linear range of 0.05−11 mg L-1 was obtained for Ag+, which is illustrated in Figure 4b. Samples with concentration of Ag higher than 11 mg L-1 or less than 0.05 mg L-1 should be diluted or enriched prior to HR-PAD analysis, respectively. The proposed method provided a limit of detection (LOD) of 0.01 mg L-1 (calculated from 3sd/slope, where sd is the standard deviation of 11 repeated measurements of a blank solution). The reproducibility of the proposed method was evaluated from 12 replicate measurements of various concentrations of Ag+. As can be seen from Figure S11 of the SI, RSDs (n=12) of 4.5% and 2.2% were obtained for 1 mg L-1 and 7 mg L-1 of Ag+, respectively. A comparison of LODs achieved using the proposed method and several alternative analytical methods is summarized in Table S3. Results show that the LOD provided by the proposed method is comparable to, or superior to those obtained using conventional techniques.

Where Eadsorbate/cluster is the total energy of the cluster with adsorbed metal ions, Ecluster is the energy of the bare cluster, and Eadsorbate is the energy of free Ag+ or other metal ions. In order to investigate the interference effects, Zn2+, Hg2+ and Pb2+ were selected for this calculation. The greater the absolute value of Eads implies a higher adsorption energy and stronger interaction between CdTe and tested metal ions. Consequently, the results show that the adsorption energy of Ag+ on (CdTe)33 cluster is -45.1 kcalmol-1, which is the most negative among the four calculated metal ions. The adsorption of Pb2+ (-20.4 kcalmol-1) is weaker than that of Ag+, but stronger than that of Zn2+ (-5.5 kcalmol-1) and Hg2+ (-2.7 kcalmol-1) (Figure 3). Observing the configuration of (CdTe)33-Ag+ complex, we find that the deformation occurs in the region where Ag+ interacts with (CdTe)33. Ag+ moves into the interior of the (CdTe)33 cluster and has a strong trend to bond with Te, while the length of Ag-Te is shortened to 2.778 Å . But the adsorption of Pb2+, Zn2+ or Hg2+ on the (CdTe)33 is quite different. The distances between these metal ions and Te is about 3.2-3.3 Å , much longer than the Ag-Te length, and the configuration of (CdTe)33 is almost unchanged. These calculation results indicate that the interaction between CdTe and Ag+ is much stronger than that between CdTe and other metal ions. Ag+ is likely to enter the CdTeQDs and produce further exchange reactions. Stability and Uniformity of CdTe QD Inkjet-Printed Paper Strips. The stability of the CdTe QDs inkjetprinted paper strips was evaluated by comparing the height of fluorescence band quenching obtained using a freshly prepared strip with one stored at room temperature for six months. Analytical results are summarized in Section 8 of the SI and show that the stability of the paper strip was good for the speciation analysis of Ag+ and AgNPs even after as long as six months storage. The uniformity of the CdTe QDs inkjet-printed paper were also investigated and described in Section 9 of the SI. The results (Figure S10) shows that the uniformity of the CdTe QDs inkjet-printed paper is much better than that obtained using the test paper by direct immersing into CdTe QDs solutions. Moreover, the mass of QDs consumed in the proposed method (0.8 mg CdTe QDs for 60 pieces of paper strips) was much lower than that required in the previously reported method based on direct immersion (1 mg for 5 pieces of paper strips)24. These results demonstrate that the proposed method not only significantly minimizes consumption of QDs but also improvesthe uniformity of the QDs distribution on the paper strip, thus alleviating the hazards of release of toxic CdTe QDs to the environment and improving analytical performance.

Figure 3. Structures of (CdTe)33 clusters and the adsorptions of Ag+, Pb2+, Zn2+ orHg2+ on (CdTe)33 clusters.

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products (Figure S12 of the SI). The results are summarized in Figure 5b, Figure 5c and Table 2, which show that only Ag+ was detected in surgical dressing, whereas AgNPs were found in all other tested products. In order to prove the correctness of the results, these samples were also characterized by LC-ICP-MS and TEM, as shown in Figure 6 and Figure S13-S15. The TEM images and chromatograms of LC-ICP-MS show that all of the tested products contain AgNPs. However, Ag+ was only found in surgical dressing products. These results agree well with those obtained by HR-PAD. It is worth noting that total silver in these products was also determined by conventional LC-ICP-MS after digestion with HNO3. The results in Figure 6, Figure S13-S15 show that no peak of AgNPs was found after digestion of products, implying that the complete digestion of AgNPs contained in products could be obtained. More importantly, the obtained total silver values agree well with those measured by LC-ICP-MS, demonstrating the accuracy of the HR-PAD which offers great potential for the equipment-free, simple and inexpensive identification and speciation analysis of Ag+ and AgNPs in river water samples and commercial products. Compared with LC-ICP-MS, the proposed method significantly reduces sample analysis time and eliminate the expensive instruments.

Figure 4. (a) the height of fluorescence band quenching by various concentrations of Ag+, obtained under a 365 nm UV-lamp excitation; and (b) the corresponding calibration curve obtained in the presence of various concentrations of Ag+.

Detection of AgNPs in Real Samples. The accuracy of the HR-PAD was initially validated by analyzing Ag+ in four river water samples collected from a local river of Chengdu city. Results are summarized in Figure 5a and Table 1, which show that direct analysis of Ag+ in these water samples cannot be undertaken because their endogenous concentrations of Ag+ are far below the LOD of the proposed method. Spike recovery was thus utilized to evaluate this practicability and found to be the range of 94%−115%.

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Table2. Analytical Results for Ag+ and AgNPs in Antibacterial Products

Table1. Analytical ResultsforAg+andAgNPs in River Water Samples

Sample

Directly detecteda,b, mg L−1

T-1

< 0.01

Detected after digestiona,c, mg L−1 0.51 ± 0.07

T-2

< 0.01

0.53 ± 0.13

AgNPsa, mg L−1

Detection by LC-ICPMSa,d, mg L−1

0.51 ± 0.07

0.59 ± 0.03

0.53 ± 0.13

0.56 ± 0.07

T-3

< 0.01

0.62 ± 0.22

0.62 ± 0.22

0.71 ± 0.11

T-4

< 0.01

0.69 ± 0.09

0.69 ± 0.09

0.72 ± 0.21

G-1

< 0.01

1.02 ± 0.10

1.02 ± 0.10

1.11 ± 0.12

G-2

< 0.01

3.19 ± 0.20

3.19 ± 0.20

3.45 ± 0.14

G-3

< 0.01

6.68 ± 0.35

6.68 ± 0.35

7.03 ± 0.09

Sample

Detected, mg L−1

Added, mg L−1

Founda, mg L−1

Recovery, %

S

6.68 ± 0.06

7.11 ± 0.09

0.43 ± 0.23

6.74 ± 0.08

River-1

< 0.01

1.0

0.94 ± 0.48

94

B-1

< 0.01

0.58 ± 0.16

0.58 ± 0.16

0.67 ± 0.13

River-2

< 0.01

1.0

1.05 ± 0.95

105

B-2

< 0.01

0.69 ± 0.19

0.69 ± 0.19

0.74 ± 0.02

River-3

< 0.01

1.0

1.10 ± 0.43

110

B-3

< 0.01

0.78 ± 0.08

0.78 ± 0.08

0.88 ± 0.04

River-4

< 0.01

1.0

1.15 ±0.27

115

B-4

< 0.01

1.23 ± 0.06

1.23 ± 0.06

1.37 ± 0.16

a Mean

and standard deviation of results (n = 3).

T, textile; G, gynecological lotions; S, suegical dtessing, and B, baby product. a Mean and standard deviation of results (n = 3); b detection of Ag+; c detection of total silver by HR-PAD; d detection of total silver by LC-ICP-MS.

The purpose of this work was to develop a disposable and instrument-free method to meet the requirements for typical use by a family or developing countries/regions for the in-situ identification and speciation analysis of Ag+ and AgNPs in commercial products. Therefore, the applicability of the HR-PAD was further demonstrated by the speciation analysis of Ag+ and AgNPs in 13 commercial

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Analytical Chemistry determination of biomarkers of disease, which will further advance point-of-care testing.

Figure 5. The height of fluorescence band quenching obtained by analyzing various samples. (a) analyses of Ag+ in river water (R-1, R-2, R-3 and R-4) and analyses of spiked Ag+ in the river samples (R-1', R-2', R-3' and R-4'); analyses of commercial products (T, textile; G, gynecologicallotions; A, surgical dressing; B,baby product.) before (b) and after (c) dissolution.

Figure 6. Characterizations of textile samples using LC-ICP-MS and TEM.

CONCLUSION

ASSOCIATED CONTENT

A CdTe QD inkjet-printed PAD with height-readout was developed for the sensitive visual and instrument-free identification and speciation analysis of Ag+ and AgNPs in river water and commercial products. An HR-PAD array with 24 analytical paper strips can be utilized as an analytical platform to significantly improve sample throughput. Since the sensitivity is only dependent on the sample volume absorbed into the paper strip, sample consumption can be remarkably reduced to 100 μL, thus providing potential for visual quantification of Ag+ and AgNPs in limited volume samples. Compared to conventional methods used for speciation analysis of Ag+ and AgNPs, the proposed methodology is straightforward, inexpensive, sensitive, and eliminates expensive and sophisticated instruments, enabling users to conduct visual detection in the field or at home even if the user is colour-weak or colour-blind. Most interestingly, AgNPs have been widely used as labels for immunoassay and further used to sensitively determine biomolecules. Therefore, the proposed HR-PAD may offer great potential for highly sensitive

The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. The operation conditions for HPLC-ICP-MS; the synthesis of CdTe QDs; characterizations of the CdTe QDs inkjet printed on paper before and after cation exchange reaction with silver ions; experimental conditions for the detection of Ag+; feasibility of speciation analysis of Ag+ and AgNPs; test for the interference from various coexistent ions; density functional theory (DFT) calculations; the lifetime of the inkjet-printed paper strip; A comparison of uniformity between the paper strips prepared by direct immersion and inkjet-printing; reproducibility of inkjet-printed PAD; A comparison of LOD achieved using the proposed method and other analytical methods; AgNPs contained commercial products; sample analysis by LC-ICP-MS and TEM.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C. B. Zheng); [email protected] (K. L. Xu). Fax and Phone: +8628 8541 0528.

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Author Contributions The manuscript was written by contributions from all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge the National Nature Science Foundation of China (No.21529501, 21622508 and 21575092) for financial support.

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Disposable Paper-based Analytical Device for Visual Speciation

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