ZnS-AgInS2 Fluorescent Nanoparticles for Low Level Metal Detection


Dec 1, 2015 - Figure 1. Schematic of generic ligand-capped fluorescent ... CuCl2, FeCl3, HgCl2, K2CrO7, KCrO4, MnCl2, and Pb(NO3)2 were ... and then m...
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Chapter 10

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ZnS-AgInS2 Fluorescent Nanoparticles for Low Level Metal Detection in Water Lee R. Cambrea, Courtney A. Yelton, and Heather A. Meylemans* Naval Air Warfare Center, Weapons Division, 1 Administration Circle, China Lake, California 93555, U.S.A. *E-mail: [email protected]

A method for the fluorescence detection of metal ions, polyatomic ions, and other environmental hazards has been developed utilizing ligand functionalized ZnS-AgInS2 fluorescent nanoparticles (ZAIS). Synthesis of the relatively non-toxic, air- and water-stable ZAIS nanoparticles (NPs) has been optimized. Charge transfer between a target molecule and the ZAIS NPs is readily identified by a fluorescence quenching allowing for a fast, simple, visual detection system without the need for expensive analytical instrumentation. Analytes of environmental interest such as Cu2+, Hg2+, and Cr6+ have been detected in concentrations as low as 500 nM (~1 ppm). The specificity for analyte binding in these NPs does not depend on ligand binding interactions specific to that particular metal as is usually seen. In the case of the NPs reported, the ligand is held constant and the selectivity appears to be dependent on the properties of the NP core.

Introduction The ability to identify heavy metal contamination in a variety of water sources, quickly and inexpensively, would greatly help in many different circumstances. Metal contamination in storm water runoff and near shipyards is of great interest for protection of our environment. These types of contamination occur at discrete time points, such as during waste-water release events or a heavy rainfall, with a limited window to identify the problem before the sample is diluted into the

Not subject to U.S. Copyright. Published 2015 by American Chemical Society In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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main water stream. Many pollution problems could be mitigated if an easy, in-situ analytical method existed to indicate the presence of contaminated water. Currently, the state of the art technique for metal detection in water is inductively coupled plasma-mass spectrometry (ICP-MS) which requires samples to be gathered and sent to a laboratory for testing. Although a very accurate and quantitive method, this method has several drawbacks. The largest drawback is the size and expense of the instrument itself. ICP-MS is not a field portable technique and therefore samples must be collected and transferred back to the laboratory for analysis, a time consuming task. Samples must also be free from particulates to avoid disrupting flow or blocking the nebulizer. Additionally, continuously running samples with high salt concentrations (like seawater) can eventually lead to blockages. These blockages can be avoided by diluting samples but this begins to affect detection limits and takes time and careful laboratory work. Fluorescent nanoparticles (NPs), particularly quantum dots, like those shown in Figure 1, have received a lot of attention lately for their potential application in many areas from new flexible or brighter displays, to higher efficiency solar panels, and even advanced bioimaging techniques (1, 2). There has been some exploration into using these nanoparticles for sensing applications, as their fluorescence intensity has been shown to depend on environmental conditions (3, 4).

Figure 1. Schematic of generic ligand-capped fluorescent nanoparticle showing the fluorescence mechanism where light (hν) excites an electron from the valence band (VB) to the conduction band (CB). The loss of energy from the excited electron results in the observed fluorescence. 196 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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For example, the presence of metal ions in solution has been shown to influence NP fluorescence either through a quenching or an enhancement of the NP fluorescence. There are several proposed mechanisms for these interactions but the most common mechanism stems from an interaction of the metal ion with a specialized ligand to create a new complex that influences the emission (5, 6). Specifically, the ligand may combine with the metal ion leaving behind a surface defect on the NP which leads to quenching of fluorescence. This quenching process allows for a visual confirmation that a metal ion is present. If the ligand and NP are properly matched it may be possible to quench the fluorescence in the presence of one or a few select metals. This selective quenching could lead to an instant-read visual test to look for the contamination of metal ions and polyatomic ions, real-time, in relevant environmental samples. These interactions with metal ions have been shown in several types of fluorescent nanoparticle systems. The most common of these NP systems are made with cadmium and either tellurium or selenium (6). They also frequently use thiol-containing ligands such as glutathione (GSH), L-Cysteine (Cys), mercaptoacetic acid (MAA), mercaptopropionic acid (MPA), or mercaptosuccinic acid (MSA) which aide in solubility as well as metal ion affinity. While some of these systems show great selectivity and sensitivity for metal ions with detection limits as low as 10-11 M (7), they are limited to laboratory use due to the toxicity of the NPs themselves. In contrast, the nanoparticles in this particular study lend themselves well to testing in a non-laboratory environment using zinc, silver and indium to create relatively non-toxic, air and water stable fluorescent nanoparticles for analyte detection in water. The first reported synthesis of ZnS-AgInS2 (ZAIS) nanoparticles was in 2007 as an observation of color-adjustable luminophores with relatively low toxicity compared to previous nanoparticles made from metals such as Cd, Te and Se (8). The applications for these nanoparticles quickly became focused on bioimaging for the same toxicity reasons, and a series of papers has been published reporting the use of ZAIS nanoparticles and quantum dots for biological sensing applications (9–12). In this work, the focus is to study the interaction of the ZAIS NP system with a variety of environmentally relevant analytes, including both metal cations and polyatomic metal containing anions. By synthesizing a series of NPs with identical non-specific ligands, it should be possible to determine if any metal specificity can be gained strictly from interaction with the NP core. Ultimately, this selectivity could then be used to create a test that could easily be performed in the field during an operation (construction, maintenance, repair, general industrial processes, etc.) without needing expensive analytical equipment or pretreatment of samples before analysis. To accomplish this goal, we need to have a fluorescence change strong enough that it is visible by eye. Then, the presence or absence of a particular analyte can be determined by simply adding a few drops of the NP solution into a sample of the water to be tested and use a UV flashlight to determine the presence or absence of specific contaminants. This test would work as a first line of defense for field operations to provide a presumptive analysis. If contamination is identified, samples would be sent to a laboratory to undergo further confirmatory testing before the field work could continue. 197 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Experimental

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General All reagents and materials from commercial sources were used as received. The solvents for synthesis and analysis along with the ligand dodecylamine, the sulfur source diethyldithiocarbamate, and the metals Al(SO4)3·18H2O, CoCl2·6H2O, CuCl2, FeCl3, HgCl2, K2CrO7, KCrO4, MnCl2, and Pb(NO3)2 were purchased from Sigma-Aldrich. The metals Zn(NO3)2, AgNO3, In(NO3)3 used in the nanoparticle synthesis were purchased from Strem Chemical. Solutions of metal ions and polyatomic anions were made by preparing a stock solution with a concentration of 5 mM and then making serial dilutions to create concentrations of 0.5 mM, 50 µM, 5 µM and 500 nM (~1 ppm). EDS spectra of the samples were taken at 20 kV accelerating voltage on a Zeiss EVO50-EP SEM (scanning electron microscope) with an EDAX energy dispersive spectroscopy (EDS) system. Major peaks (measured in keV = kiloelectron volts) were identified, which correspond to elements present in the samples. SEM images were also taken to show representative sample features.

Synthesis The nanoparticles were synthesized as previously reported with a few modifications as outlined (8, 11). A powder was made using the four elements (zinc, silver, indium, and sulfur) by adding 2-4 mmol of metal in a ratio of 2*(1-x)Zn, xAg, xIn and 2-4 mmol of sulfur in 20 mL water and stirring at high speed for 5 minutes. The starting materials used are Zn(NO3)2, AgNO3, In(NO3)3 and diethyldithiocarbamate (as the sulfur source) and they were added as outlined in Table 1 for the samples described throughout. The powder was then filtered through a medium porosity frit and washed with water and methanol. The powder was dried overnight in the oven at 40 ºC. After drying overnight, 50 mg of the powder was placed in a flask and heated to 180 ºC. After heating for 30 minutes, 1 ml of dodecylamine ligand was added and the mixture heated for a further 3 minutes. The resulting liquid was centrifuged at 5000 rpm for 15 minutes. The supernatant was removed and washed with methanol and centrifuged again at 5000 rpm for 15 minutes. The supernatant was removed and the precipitate dissolved in 10 mL of chloroform (CHCl3) or dimethyl sulfoxide (DMSO) depending upon the desired solvent. The solvent must be added immediately. If the NPs are allowed to completely dry before the addition of solvent, aggregates form and it is very difficult to get the NPs re-suspended even after significant sonication steps. Average yields are approximately 50% (~100 mg). Final samples are made using absorbance intensity matching to maintain consistent concentrations throughout the experiments. Initial particle size analysis indicates that the NPs range from 2-10 nm depending upon composition and batch. 198 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Table 1. Sample Preparation for NP Synthesis Sample ID

Ratio used Zn:Ag:In

Zn (mmol)

Ag (mmol)

In (mmol)

S (mmol)

NP1

0:1:1

0.00

0.63

0.63

2.49

NP2

0.6:0.7:0.7

0.38

0.44

0.44

2.49

NP3

1.2:0.4:0.4

0.75

0.25

0.25

2.49

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The ratio of sulfur was held to a constant 2x mmol of the total mmol of metal complexes added.

Absorption and Emission Spectra All spectroscopic data were obtained on samples dissolved in either CHCl3/ CH3CN or DMSO/H2O solvent systems. Absorption spectra were measured with a Shimadzu UV-3600 UV-Vis-NIR spectrophotometer. Absorbance intensity at 395 nm was used to ensure that different samples of quantum dots were held at a consistent concentration. Emission spectra were collected at 395 nm excitation wavelength using a Horiba Jobin-YvonFluoroMax-P fluorimeter. Samples for blank (neat solvent) and background (solvent plus analyte) emission scans were collected using 3 mL of the desired solution. Emission measurements on solvent blanks and analytes in the absence of nanoparticles revealed no signals other than the expected Raman lines of the neat solvent. Samples used to measure the response of the nanoparticle fluorescence in the presence of analyte were prepared by using 3 mL of the desired analyte solution and adding 1 mL from a stock solution of nanoparticles. Emission spectra were then collected immediately after mixing of the two solutions. Data reported in the charts and tables are reported at the emission maximum (λmax) for each of the neat NP samples. The data are reported in terms of E/E0, where E is the intensity of the sample emission with analyte present and E0 is the intensity of the neat sample emission.

Results and Discussion ZAIS NPs were synthesized by way of previously published procedures with some modifications (8, 11). For this initial set of samples, the amount of indium and silver was kept the same (x) and the amount of zinc was varied as 2*(1x). The amount of sulfur in the samples was always equimolar with the total metal concentration. Three different samples were synthesized, characterized, and tested, Table 2. 199 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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Table 2. Composition and Emission Data (Excitation Wavelength (λex) = 395nm) Sample ID

Ratio of metals Zn: Ag: In

λmax (nm)

Quantum Yield

FWHM (nm)

NP1

0:1:1

669

0.22

155

NP2

0.6:0.7:0.7

577

0.16

124

NP3

1.2:0.4:0.4

548

0.23

160

Each of the samples was characterized using EDS, UV-Vis and fluorescence (Figure 2 A-D). While EDS is not a quantitative technique, it shows that all of the expected elements (identified by peak labels) are present in the samples (Figure 2A and 2B). UV-Vis spectra were collected for each of the samples from 250-800 nm and as expected the absorbance for all three samples lies in the UV region of the spectrum below 450 nm (Figure 2C). The samples were each excited at 395 nm and the emission spectra were collected from 425 nm-775 nm (Figure 2D) with emission maximum (λmax) and full width half maximum (FWHM) reported in Table 2. The reported FWHM of the emission peak is very large compared with other traditional NP systems such as CdSe (FWHM less than 30 nm), but this is consistent with previous reports of ZAIS NPs where similar FWHM values are seen (8). Related to this the quantum yields of the NPs are also in line with those previously reported and may be slighter higher than some other similar NPs due to the larger FWHM (8). Comparisons of relative fluorescence intensity, in the presence of analyte, were determined at λmax throughout. The nanoparticles were all initially synthesized with the same ligand, dodecylamine. The ligand was chosen to have limited/no interaction with the analytes in solution allowing an investigation into the effects of the nanoparticle’s metallic core composition. Further work is on-going to investigate the effect of metal specific ligands and selective binding based on ligand characteristics. Surprisingly, initial testing with the dodecylamine nanoparticles showed that selectivity might not be entirely ligand dependent. It should be noted that all of the experiments done with these NP were performed open to the air and under normal room light. There was no noticeable difference in fluorescence of the stock solutions in these conditions for greater than 18 months. This suggests that these NP are very air-stable and can be used off the shelf for quite some time. 200 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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Figure 2. A-D. EDS spectra (2A-B) show the presence of elements in a sample. Elements of C and O are present from the ligand and residual counterions, respectively. A. EDS spectra of NP1 shows the absence of Zn. B. EDS spectra of NP2 shows the addition of Zn and all other components the same. C. UV-Vis absorbance spectra for the three NP samples. D. Emission spectra for the three NP samples resulting from excitation at 395 nm.

An initial set of experiments was performed with metal ions in acetonitrile (CH3CN) solutions, and the NPs dissolved in CHCl3. These solvents were chosen because it was observed that, under these conditions, the nanoparticles do not aggregate or precipitate from solution. Generally there was no significant change to the fluorescence upon exposure to the metal ions; however, the exception to this was the preference for Cu2+ ions over the other metals. Even with Cu2+ concentrations as low as 500 nM (~1 ppm) the NP3 fluorescence was completely quenched (Figure 3). This was not an anticipated result, and it is even more curious when one considers the lack of reactivity between Cu2+ by the other samples, NP1 and NP2. With all NP samples having the same ligand, it is suggested that the NP core has a role in the interaction with the metal ions. Additionally, the concentrations of the metals in the nanoparticle core are important for selectivity. 201 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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Figure 3. Emission spectra of NP3 in CHCl3/CH3CN with 500 nM (~1 ppm) of Fe2+, Co2+, and Cu2+ metal ions. The spectra show a clear quenching of the fluorescence in the presence of Cu2+.

In an attempt to understand the interaction of the metal core and resulting selectivity, additional metal ions and some polyatomic ions were tested. For this second set of analytes, the solvent system was altered for two reasons. First, the analytes were more soluble in aqueous solutions and second, it was important to use a solvent system more relevant to the ultimate application. A set of NP samples were suspended in DMSO, and the absorbance and emission were once again measured to ensure that there were no solvent effects taking place in the system. Water was then added to the DMSO samples to ensure that water alone did not quench the fluorescence. The absorbance and emission spectra remained consistent, which indicates that there is no significant interaction between the NPs and DMSO or water. Analytes were dissolved in deionized water to known concentrations before testing. A series of analytes was selected based on known environmental hazards or general interest. Specifically hexavalent chromium (which manifests as chromate and dichromate in water) is known to be toxic and is highly regulated. In addition to general health concerns with chromate exposure, the Navy has a particular interest in hexavalent chromium because it was used in chromate conversion coatings as a corrosion inhibitor and the Navy is still trying to eliminate it from the fleet. All solutions were prepared from chloride salts, except for the chromate and dichromate, lead, and aluminum samples, which were potassium, nitrate, and sulfate salts, respectively. Each of these metals were made and tested in water at five different concentrations made via serial dilution in a range of 5 mM – 500 nM. Samples used to measure the response of the nanoparticle fluorescence in the presence of analyte were prepared by using 3 202 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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mL of the desired analyte solution and adding in 1 mL from a stock solution of nanoparticles. Emission data was then collected immediately after mixing of the two solutions. In order to be applicable for a test that can easily be performed in the field, without needing expensive analytical equipment, the fluorescence change must be distinct enough to be detected by the naked eye. Figure 4 shows a representative visual change in fluorescence due to quenching in the presence of an analyte. The measurements are reported by comparing the resulting fluorescence intensity after addition to analyte solution (E) to the initial fluorescence intensity (E0) before analyte exposure. Depending on the sensitivity of the NP to a particular metal the sample can appear to be unreacted (E/E0 of 0.8-1.0), completely reacted and no fluorescence detected by eye (E/E0 of 0.2 or less) or partially reacted where fluorescence is visible but weaker than the starting intensity (E/E0 of ~0.2-0.8). To categorize these three regimes is fairly straightforward, by eye, without the need for additional instrumentation to actually measure the fluorescence.

Figure 4. Visual representation of a NP sample at different fluorescence levels to show how easily it is to see the fluorescence quenching by eye. Labels on each vial relate to the y-axis in Figures 5-7 where emission is reported as E/E0. By eye fluorescence is completely quenched (off) at E/E0 ≤ 0.2, partially quenched at E/E0 ~ 0.2-0.8 (dim) and no quenching when E/E0 ~ 0.8-1.0 (bright). Laboratory testing of the three NP samples with various analytes was conducted by visual inspection and then fluorescence measurements were taken to determine the fluorescence remaining compared to the starting NP sample (E/E0). The predicted outcome of these tests was as the concentration of analyte in solution increases, the fluorescence of the NP sample would decrease (quenching). Or, alternatively that the NP will have no interaction with the analyte regardless of concentration and therefore the fluorescence would remain unaffected. As illustrated in Figure 5, NP1 showed the expected reaction trend. It is observed that the fluorescence becomes more quenched as the concentration of analyte is increased, for most of the analytes. There was, however, one exception to this trend, seen with the polyatomic anion dichromate. The reaction of NP1 203 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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with dichromate led to a complete quenching of fluorescence at concentrations as low as 500 nM. Interestingly, chromate despite being the same metal (in the same oxidation state) did show some fluorescence at these lower concentrations from 50 µM to 500 nM, while concentrations higher than 50 µM quenched the fluorescence in this sample. These results were unexpected given the generality of the dodecylamine ligand, but may point toward ion size or overall charge having an effect on selectivity. Additionally, the interaction of dichromate with NP1 must happen quickly before the interconversion to chromate can take place. The different observations between chromate and dichromate, both of which are Cr6+ containing polyatomic anions, also indicate dissociation to free Cr6+ is not the mechanism. The other observation that was unexpected, and can be seen in the data, is that the level of quenching is not linear with these samples. There is clearly a threshold where there is little to no interaction with the NP and then the interaction is drastic and the sample is quenched. This is seen most obviously in both the Cu2+ and Hg2+ solutions. In both cases at a concentration of 5 µM or greater the sample is quenched. This characteristic could be useful in determining not only which metals are present in solution but also help to give some indication of the concentration of that metal as well.

Figure 5. Emission intensity ratios are reported for NP1 at λmax = 669 nm by comparing the resulting fluorescence intensity after addition to analyte solution (E) to the initial fluorescence intensity (E0) before analyte exposure. Concentrations of ions in each group increase from left to right in each series as 500 nM, 5 µM, 50 µM, 500 µM, and 5 mM. Three background regions labeled on the right side correspond to the visual limits outlined in Figure 4. 204 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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Results from the testing of NP2 were expected to look the same as NP1 given that the ligand surrounding the metal core was again dodecylamine. No selectivity was anticipated; yet, NP2 not only showed selectivity for certain analytes but the selectivity was different than seen in NP1 (Figure 6).

Figure 6. Emission intensity ratios are reported for NP2 at λmax = 577 nm by comparing the resulting fluorescence intensity after addition to analyte solution (E) to the initial fluorescence intensity (E0) before analyte exposure. Concentrations of ions in each group increase from left to right in each series as 500 nM, 5 µM, 50 µM, 500 µM, and 5 mM. Three background regions labeled on the right side correspond to the visual limits outlined in Figure 4.

NP2 showed a significant selectivity for mercury ions in solution. This result is the opposite of the reaction with Hg2+ seen in NP1 where mercury was the least reactive metal that was tested. For solutions containing concentrations of Hg2+ ions as low as 500 nM, the solution is completely quenched by eye for NP2. This was also true for chromate when the NP2 solution was used for the test. And, as seen with NP1 the detection of dichromate and chromate are different. In contrast to both of the other NP samples, NP3 appears to have no selectivity for any particular analyte in water. This is true despite the excellent selectivity for Cu2+ seen in the CH3CN/CHCl3 solvent system (Figure 2) for this same NP. The NP3 sample shows the most consistent response to each analyte, showing a general trend of quenching as the concentration of analyte increases (Figure 7). This may be a good configuration to use in the lab as a reference system. 205 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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Figure 7. Emission intensity ratios are reported for NP3 at λmax = 548 nm by comparing the resulting fluorescence intensity after addition to analyte solution (E) to the initial fluorescence intensity (E0) before analyte exposure. Concentrations of ions in each group increase from left to right in each series as 500 nM, 5 µM, 50 µM, 500 µM, and 5 mM. Three background regions labeled on the right side correspond to the visual limits outlined in Figure 4.

NP3 is the only sample in which all of the different ions completely quench the sample at a concentration of 5 mM or greater. Similarly, once the concentration of the analyte drops lower than 5 µM the fluorescence is visible regardless of the analyte, indicating there is no selectivity in NP3. Interestingly, all three NP samples had quite the opposite reaction when tested with cadmium (Cd2+) ions. In this case, as the concentration of cadmium in solution increased the fluorescence of the NPs was enhanced rather than quenched as seen with all other metals tested. Figure 8 shows the emission spectra of NP2 with various concentrations of Cd2+ ions. The fluorescence increases by nearly an order of magnitude when compared with NP2 in the absence of Cd2+. This order of magnitude increase is seen for all three of the NP samples in the presence of cadmium. 206 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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Figure 8. Fluorescence intensity of NP2 in the presence of Cd2+ ions. As the amount of Cd2+ is increased the fluorescence increases by an order of magnitude.

These varying results, quenching vs enhancement, suggest that different mechanisms for interaction of the analytes are at play depending upon the characteristics of the analyte as well as the NP core. Several mechanisms for both quenching and enhancement of fluorescence have been proposed previously and the most likely will be discussed here (5, 6). Most reports of nanoparticles or quantum dots selectively quenching are generally attributed to specific interactions between the ligand on the NP and the metal being tested which creates a complexed species that disrupts fluorescence (3, 4). However, given the generality of the dodecylamine ligand this is not likely to be the mechanism at play in this case. Figure 9 illustrates a couple of different possibilities for the mechanism that could be occurring in this system. Pathway A in Figure 9 shows ligand displacement where the analyte displaces the dodecylamine ligands for a more favorable interaction. This leaves an unstable quantum dot that may precipitate or agglomerate in solution leading to quenching of fluorescence. In a few cases, particularly when the concentration of analyte in solution was in the millimolar range, some precipitate could be seen in the quenched samples for all three of the NPs tested. This is in agreement with mechanism A. However, given that the same ions did not interact with all of the samples it may be more complex than this mechanism. 207 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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Figure 9. Proposed mechanisms for analyte interaction with the NP samples. Pathway A shows a quenching mechanism where ligands combine with the analyte, in solution, creating defects that lead to quenching. Pathway B outlines a case where the analyte interacts with the NP core and causes quenching through other decay pathways such as non-radiative decay. Pathway C shows a passivation mechanism where the analyte incorporates into the surface of the NP repairing defect sites and enhancing fluorescence.

Pathway B of Figure 9 shows a direct interaction between the NP core and the analyte that would lead to quenching of the fluorescence. This mechanism has been suggested only a few times and is generally defined by an enhancement in fluorescence at relatively low concentrations, followed by quenching at higher concentrations (13). While most of the selectivity in the samples studied here show a complete quenching of the NP even at low concentrations it may be that even lower concentrations need to be explored to see this initial enhancement. However, for the less quenched analytes (for example Hg2+ or Cu2+ with NP3 (Figure 7)) it should be noted that there is a significant drop in fluorescence at a particular threshold and that the quenching mechanism does not appear to be linear as is generally seen for other quenching mechanisms. This may be an indication that the NP samples studied here do indeed have this direct core interaction. Finally, pathway C of Figure 9 is the most likely to explain the interaction of all three NP samples with Cd2+. This interaction referred to as the passivation mechanism is explained by Cd ions filling defect sites on the NP surface to enhance fluorescence by minimizing non-radiative electron-hole recombination (14). This same enhancement has been seen for Zn2+ detection using CdS quantum dots, and for CuInS2 quantum dots in the presence of Cd2+ (15, 16). 208 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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Conclusions Despite the similarity of all of the samples that were prepared for this study some analyte selectivity was found. Selectivity appeared to be solvent-dependent in some cases. For example, in the case of NP3, the samples were very reactive with Cu2+ in non-aqueous solution and completely quenched with concentrations of copper as low as 500 nM. However, the same nanoparticle, NP3, showed no selectivity for Cu2+ in aqueous solution and only quenched at concentrations more than 5 mM, i.e., concentrations that are greater by more than 5 orders of magnitude. Additionally, the ratio of metals used in the nanoparticle syntheses seemed to have some influence on selectivity, as evidenced by NP1 quenching selectively in the presence of very low concentrations (500 nM) of dichromate, and NP2 quenching selectively in the presence of Hg2+ and chromate ions at the same concentration. To our knowledge, this is the first time that the chromate and dichromate anions have been explored for detection using nanoparticle sensors. Having sensors that not only detect chromate and dichromate but that may be able to selectively detect one over the other could be of great interest for environmental testing. The next step will be to look at how the NP fluorescence is affected in untreated seawater in the presence of analytes. Initial testing indicates that the fluorescence of the NPs in seawater, without added analytes, is unaffected. It is important to have a balance between sensitivity/detection limits and naturally occurring species and we hope that future work in this area will show that this is possible. While this technique is not sensitive or specific enough to replace ICP-MS, it may be a good first line of defense test that would allow for fast field determination of a contaminant. Additionally, being able to test samples without any pre-treatment and only needing a UV flashlight for visual detection makes these nanoparticle systems easy and inexpensive. Further work is underway to optimize the metal selectivity through ligand modification and streamline the testing system to allow for even easier visible detection.

Acknowledgments The authors wish to thank Ms. Madeline Kooima for synthetic explorations at the beginning of the effort. We also wish to thank the NAWCWD Core S&T program for funding this effort.

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