Ratiometric Phosphorescent Silver Sensor: Detection and

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A Ratiometric Phosphorescent Silver Sensor: Detection and Quantification of Free Silver Ions within a Silver Nanoparticles Medium Erin N. Benton, Sreekar B. Marpu, and Mohammad A. Omary ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01224 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 24, 2019

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ACS Applied Materials & Interfaces A Ratiometric Phosphorescent Silver Sensor: Detection and Quantification of Free Silver Ions within a Silver Nanoparticles Medium Erin N. Benton†, Sreekar B. Marpu†,*, Mohammad A. Omary* Department of Chemistry, University of North Texas, Denton, TX, 76203 USA ABSTRACT: Silver nanoparticles (AgNPs) have well-known antibacterial properties that have stimulated their widespread production and usage, which nonetheless concomitantly raises concerns regarding their release into the environment. Understanding the toxicity of AgNPs to biological systems, the environment, and the role that each silver species (Ag+ ions vs AgNPs) plays in that toxicity has received significant attention. One of the critical objectives of this research is the development of a reliable method that can sense and differentiate free silver ions from AgNPs, and is able to characterize silver ions leaching from the nanosilver. A number of analytical methods described in the literature that are available for sensing silver ions are costly, time-consuming, tedious, and more importantly, destroy the AgNP sample. To address these issues, a phosphorescent gold(I)-pyrazolate cyclic trinuclear complex (AuT) known to detect free silver ions was employed to detect and differentiate silver ions from AgNPs within an AgNP sample. The advantage of the proposed silver sensor is its ratiometric emission capability that undermines any background interference. The sensor exhibits a strong red emission (λmax ~ 690 nm) that - in the presence of Ag+ ions will form a bright-green emissive adduct with a peak maximum near 475 nm. The presence of AgNPs did not inhibit the silver detection and quantification ability of the phosphorescent silver sensor. In order to understand the chemical transformation of nanosilver, the leaching of silver ions from AgNPs over a period of 35 days was monitored and quantified by measuring the I/Io changes of the sensor. Furthermore, through adduct formation, the AuT molecular system was able to remediate free silver ions from the solution. The stronger affinity of the AuT complex to “sandwich” free silver ions was demonstrated in the presence of a KCl salt that is well-documented to form AgCl in the presence of silver ions. To our knowledge, this is the only ratiometric luminescence–based silver sensor able to successfully differentiate between Ag+ ions and AgNPs, sense the silver leakage from AgNPs, and remediate toxic silver ions from solution. The synthesis and characterization of this sensor is a simple, single-step process – anticipating its viability for various applications. KEYWORDS: silver nanoparticles, silver ions, silver toxicity, luminescent silver sensor, phosphorescent silver sensor, ratiometric sensing, gold trimer, gold-gold interactions INTRODUCTION One of the largest sources of silver contamination is from engineered silver nanoparticles (AgNPs). Especially in the last decade, AgNPs have become very common in many commercially-available products such as bedding, toothpaste, bandages, fabrics, deodorants, kitchen utensils, and toys -- due to their known antibacterial properties (1, 2). In addition, scientists further take advantage of the antibacterial properties of AgNPs by using them in other applications such as pharmacology, human and veterinary medicine, food industry, and water purification (3). The interest in using AgNPs as an antibacterial agent comes from the fact that certain bacteria such as MRSA are becoming resistant to antibiotics (1). The potential for silver as an alternative to antibiotics is due to the many studies showing silver’s effectiveness on a wide range of bacteria (1). The potential mechanism of silver’s antibacterial properties involves their accumulation in bacterial cells, resulting in shrinkage of the cytoplasm membrane and detachment from the cell wall (3). Therefore, DNA molecules become condensed and lose their ability to replicate (3). Unfortunately, it is a known issue that silver ions are toxic to humans because silver can be absorbed through the lungs, gastrointestinal tract, mucous membranes, and skin (4). Studies have shown that there has not been any documented beneficial/essential physiological or biochemical role for silver in the human body. Excessive silver ion intake can lead to the long-term accumulation of insoluble precipitates in the skin, eyes, and other organs -- causing various medical conditions (5). Therefore, the release of various silver species (silver nanoparticles and different silver salts) into the environment from multiple sources and applications is concerning. Understanding the toxic effects of free silver salts is arguably

straight-forward and easily-studied (6). However, studying the toxicity mechanism of AgNPs to various biological systems is not quite clear (7-10). This challenge is due to the dynamic transformation of AgNPs to silver ions upon interacting with the media. The chemical and morphological changes of AgNPs makes it difficult to understand the exact mechanism of toxicity of AgNPs in different media. Therefore, one important step would be the ability to differentiate the free silver ions leaching or the silver ions chemically transformed from AgNPs so that the toxicity specifically due to free silver ions vs AgNPs can be clearly-understood. Due to the fact that different NPs under different conditions (such as pH, size, and chemical composition) could greatly alter the physiochemical and morphological properties and thereby the toxicity of the AgNPs, it is important to characterize silver leaching in the specific environment of that particular application (2, 11). The challenge of understanding the role of different species in different media can be made very easy if each of these species could be isolated and quantified. Currently, a combination of field flow fractionation (FFF) and inductively coupled plasma mass spectrometry (ICP-MS) are adapted in a combination with various other detectors to determine size and quantification of AgNPs in aqueous matrices. Other combinations of detectors include simple UV-Vis spectrophotometer, centrifugal ultrafiltration and diffusive gradients for detection and separation of AgNPs based on their surface plasmon resonance and size (12). Additionally, there are numerous well-known approaches to quantify the silver ion concentration down to the part-per-billion (ppb) level. These techniques include atomic absorption (AA) spectroscopy, ICP-MS, and potentiometric methods based on ion-selective electrodes (13, 14). Most of these methods are time-consuming, expensive and unable to

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differentiate AgNP from silver ions (14). Also, the sample preparation needed for these methods can induce changes to the properties of the AgNPs, which introduces uncertainty in the subsequent analytical results. These AgNPs modifications can create a large gap for understanding the actual interactions of different silver species in the environment and in biological systems. However, luminescent indicators are advantageous due to their high sensitivity, rapid response, and ease of use (4, 13, 14). Nonetheless, there is still a challenge in quantifying the exact concentration of silver ions to the ppb level -- even in the presence of silver nanoparticles -- without sacrificing the AgNPs sample. This paper documents the development of an optical sensor sensitive to silver ion concentration in aqueous chitosan (CS) matrix down to the ppb range, and that uniquely identifies silver and differentiates free silver ions from AgNPs. Further, the ability of the sensor for sensing silver ions is not crippled in an AgNP medium. This differentiation between the AgNPs and silver ions is the first step towards determining the exact role of different silver species in terms of their relative contribution to the toxicity of AgNPs. Additionally, our previous work using the same system has already (15) demonstrated that the sensitivity of the sensor is not affected by the presence of other inorganic salts in the aqueous medium. In addition, this sensor is able to detect the leaching of silver ions from AgNPs over time, as well as being able to remediate these ions from solution. EXPERIMENTAL SECTION Materials Silver nanospheres stabilized in polyvinylpyrrolidone (PVP) used in this study (0.02 mg/mL; 20-nm and 100-nm diameter) were purchased from nanocomposix. The gold precursor, Au(tetrahydrothiophene)Cl (Au(THT)Cl), was synthesized by following literature procedure (16). Silver nitrate, potassium chloride, low-molecular-weight chitosan (CS), and 5-methyl1H-pyrazole-3-carboxylic acid were purchased from Sigma Aldrich and used without further purification. Physical Measurements Steady-state photoluminescence (PL) spectra were acquired with a PTI QuantaMaster Model QM-4 scanning spectrofluorometer attached with a 75-watt xenon arc lamp. pH measurements were made using a Hanna instrument HI1053B pH probe. Electronic absorption spectra were obtained with a PerkinElmer Lambda 900 double-beam UVVis-NIR spectrophotometer. Synthesis of the gold trimer (AuT) sensor The AuT cyclotrimeric complex was synthesized by adaptation of the published literature procedure (15). To begin, a 10-mL sample of dialyzed 1 wt% CS was added to a beaker. Then, 15 mg of 3-methyl-1H-pyrazole-5-carboxylic acid was dissolved in 1 mL of methanol, added to the CS and allowed to stir for 10 min. After stirring, a 240-µL aliquot of 2 M NH4OH was added to increase the pH to around 6.5. Lastly, a 5-mg sample of Au(THT)Cl was added and the solution was allowed to stir for 45 minutes. Any undissolved gold was centrifuged out, resulting in a clear solution.

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RESULTS AND DISCUSSION AuT selectivity for Ag+ ions The synthesis, characterization and photoluminescence properties of the cyclic pyrazolate trimer have been extensively delineated in our previous work (15). The changes in emission color of the Au(I)-pyrazolate trimer (AuT) from red (690 nm) to green (475 nm) in the presence of silver ions has been demonstrated in our previous work using the same complex. The factors affecting the green emission at 475 nm in the presence of silver and its detection limits in different polymer concentrations were also clearly documented. Additionally, we have shown that the selectivity and sensitivity of the trimer for silver ions is not affected by the presence of numerous other inorganic salts. Realizing the significance of differentiating the presence of silver ions vs AgNPs, the first step was to ensure that the sensor only responds to free silver ions (monovalent Ag+ form) and not the AgNPs (zero-valent Ag(0) form) in solution. The photoluminescence (PL) spectra and emission color changes of the samples in Figure 1A demonstrate the capability of the sensor to selectively detect free silver ions and differentiate those silver ions from AgNPs. The figure shows that upon addition of 0.01 mg of 100 nm AgNPs to AuT, a very minor change in the PL spectrum of the AuT complex is observed. The rise of a weak emission shoulder at 475 nm is due to the interaction of AuT with small amounts of free silver ions in the AgNPs solution. Comparatively, the results show that upon addition of the same concentration of Ag+ ions (0.01 mg), a distinct green emissive peak is formed which is 4x more intense than the AgNPs emission response. The inset pictures clearly show that the samples containing silver ions vs AgNPs are easily differentiated when excited with a hand-held UV lamp. This represents strong evidence that the AuT sensor exhibits formation of a green-emitting sandwich adduct described in ref 15. This allows for easy differentiation and quantification of free silver ions from AgNPs in solution without additional sample preparation. From our previous studies, we have already established that the peak maxima of Ag+-AuT sandwich adduct’s green emission exhibit a minor shift from 475 nm to 500 nm at higher concentrations of free silver (15). Upon close comparison of the silver adduct peak maximum in both the AuT+AgNPs and AuT+Ag+ solutions (Figure 1A), a noticeable shift from 475 nm to 500 nm is observed. The 475 nm peak maximum corresponding to the silver adduct peak in the AgNP solution indicates the presence of an insignificant amount of free silver ions in the AgNP solution. This is expected based upon the stability and continuous chemical transformations of AgNPs in solution. However, in the presence of free silver ions of the same concentration, the silver adduct exhibits an emission peak maximum at 500 nm with a much higher PL intensity, indicating a much higher concentration of free silver ions. This result is very significant because it suggests AuT’s selectivity to free silver ions vs AgNPs. Additionally, it suggests the ability of the sensor to detect very low concentrations of silver ions in an NP medium -- undisturbed by the presence of AgNPs. The blue color noticed in the inset picture (Figure 1) is the background interference arising from the 1wt% CS polymer that mediates the in-situ synthesis of AuT. The inset pictures and the spectra clearly show that the CS polymer does not affect the silver sensing of the AuT system. In order to more precisely 2


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ACS Applied Materials & Interfaces understand the interference of AgNPs on sensing free silver ions, two different titration experiments were conducted, as shown in Figure 1B. The black line in Figure 1B represents data for the addition of free silver ions to an existing aliquot of AuT/AgNPs solution, while the red line represents the addition of AgNPs to an existing solution of AuT/Ag+. The x-axis clearly shows that in both cases, the same amount of silver (free silver or AgNPs) was added to the AuT complex. The continuous increase in the intensity of the 475-nm peak for the free silver titration graph (black line), clearly indicates the continuous complexing of AuT with free silver ions even in the presence of AgNPs. On the other hand, except for the first Ag+ addition, the 475 nm PL signal remains constant during the AgNP titration experiment. This result clearly indicates the sensor’s selectivity for free silver ions even in the presence of AgNPs. If this sensor were equally sensitive to both Ag+ ions and AgNPs then both the black and red lines would overlap with each other. The weak response of the sensor to the AgNPs addition (red line) could be due to the presence of free silver ions in AgNPs solution, as noticed in Figure 1A.

Sensing Ag+ ion leaching from AgNPs Now that it has been established that this sensor only detects free Ag+ ions in solution, the ability of the sensor to determine the leakage/leaching of silver from AgNPs in solution over time was evaluated. The chemical transformation of AgNPs into various silver species is one of the most challenging aspects for clearly-understanding the toxicity of AgNPs in the environment. Information regarding the release or leakage of silver ions from AgNPs would be extremely helpful in this respect. Figure 2A illustrates the AuT sensing of silver leaching/leakage from AgNPs over time. The same concentration of 20 nm AgNPs was titrated into the sensor on day-1, day-21, and day-35. Between experiments, the AgNPs were stored at room temperature and under ambient light to promote leaching/leakage of silver ions. Based on Figure 2A, the increase in I/Io, indicates the leakage of silver ions from AgNPs over time. In addition, changes in the AgNP properties (surface plasmon resonance (SPR), stability, and aggregation) were monitored using UV-Vis data as collected throughout the experimental time period. Figure 2B shows the changes in the SPR of the 20 nm AgNPs sample on day-1, day-21, and day-35, respectively. Figure 2B clearly shows a decrease in absorbance of the AgNPs that would result either from a partial transformation of AgNPs to silver ions or from the aggregation of the AgNPs. The increase in the full-width-at-half maximum (FWHM) of the SPR peak from 2757 cm-1 to 3122 cm-1 over the course of the experiment clearly indicates that AgNPs were aggregating. In addition, the shift in peak max from 394 to 399 nm is another indication of aggregation. This is to be expected since the AgNP samples were not stored properly in order to maximize their stability, given the necessity to accelerate their aggregation or decomposition for this study. However, from Figure 2A the increase in silver ion concentration due to the transformation of AgNPs is clearly indicated by the increase in I/Io. Based on these data, it can be concluded that the AgNP samples were aggregating as well as leaching silver ions. Further studies are warranted to determine how this aggregation affects silver ion leaching. The greatest advantage of this sensor, we believe, is the ability to use simple UV-Vis data to understand the physical and chemical changes of the AgNPs. In the absence of such a straight-forward and cost-effective optical sensor, the analysis of silver leakage would require the use of ICP-MS or AA instrumentation with rigorous sample preparation that compromises the sample quality in terms of representing the identity of its silver species constituents. These data clearly indicate the ability of the sensor to detect the leakage of silver ions from AgNPs. Also, in combination with simple UV-Vis spectroscopy, the sensor can help one understand the aging and decomposition of AgNPs, which involves the aggregation of AgNPs and/or release of free silver ions.

Figure 1: A) Spectra of AuT alone, AuT+Ag, AuT+AgNP, and 1wt% CS. Inset photographs were taken under a hand-held UV lamp under both long and short wavelength (254 nm and 365 nm) associated with the spectra. B) Black line: One addition of AgNP with subsequent additions of Ag+ ions. Red line: One addition of silver ions with subsequent additions of AgNPs. The Ag+ ions were prepared at the same concentration as the AgNPs. For all PL, spectra λex=320 nm and λem=475 nm. 3



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AgNP characterization techniques such as UV-Vis. Specifically, AgNPs have very high extinction coefficients; consequently, sensing small changes in AgNP concentration is very difficult. In addition, the dialysis water was also tested for the presence of silver ions. We were unable to sense any silver ions within the dialysis water itself due to the extremely small concentration of silver ions removed from AgNPs. Therefore, sensing such a small concentration of silver ions would prove challenging even to extremely sensitive techniques such as AA. These data illustrate the advantage of our sensor in that we are able to detect small changes in silver concentration within an AgNP medium where common techniques like UV-Vis and AA are unable to.

Figure 2: A) AuT sensing the leaching of silver ions from 20nm AgNPs over 35 days. The control spectrum of the silver ion titration was done at the same concentration as the AgNP (0.02mg/mL). B) UV-Vis spectra of 20-nm AgNPs. Spectra were collected the same day the data in A were taken. Inset chart shows the change in FWHM and peak maximum of the AgNP UV-Vis spectra over 35 days. All PL spectra were collected with λex=320 nm and λem=475 nm. Next, we investigated the sensor’s ability to detect and differentiate the presence/absence of free silver ions after dialyzing the AgNPs (Figure 3). After sensing the initial silver content in the AgNPs (black line in Figure 3A), the AgNPs were then placed in dialysis tubing (3000 Da) and dialyzed for 7 days. After this dialysis, the silver content of the AgNP solution was retested and showed a drastic decrease in the sensor’s response -- indicating the presence of very minute quantities of free silver ions that were not removed during dialysis (Figure 3A). We believe that there is a dynamic equilibrium between AgNPs and free silver ions; therefore, further removal of silver ions was not possible even by dialysis. Along with the PL data, UV-Vis spectra were also acquired for the AgNPs before and after dialysis. Initially, the UV-Vis data could seem to apparently contradict the PL data; however, this is not the case. Specifically, according to the UV-Vis data, there was no change in the AgNP concentration, size or stability before and after dialysis. However, these PL data show rather clearly that the sensor has the potential to identify very small concentrations of free silver ions that are associated with the AgNPs. These data show the advantage of our silver sensor over conventional

Figure 3: A) AuT sensing of silver ion concentration of 100-nm AgNPs before and after the AgNPs have been dialyzed for 7 days. B) UV-Vis spectra of the 100-nm AgNPs before and after dialysis. Spectra were collected the same day the data in A were taken. For all PL spectra, λex=320 nm and λem=475 nm. AuT Ag+ remediation from solution Based on literature data on similar cyclic systems, we strongly believe that these Au(I) systems have a strong affinity to form “sandwich” adducts with heavy metals within cyclic trimer rings (16, 17). It is well established that these complexes are luminescent due to the formation of cyclic dimer-of-trimer units (18-20). The luminescence of this complex changes from red to green upon formation of a “sandwich” complex with free Ag+ ions. Taking advantage of this chemistry, we wanted to determine if this sensor could not only sense silver ions in 4


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ACS Applied Materials & Interfaces solution but whether it could also have the potential to remediate/extract those ions from that solution. Such a sensor could be utilized for both sensing and remediation applications. Silver ions have been shown to be toxic for various biological systems. Therefore, it would be highly advantageous to not only sense their presence but also remediate them from the environment. Additionally, the simple and straightforward synthesis of this sensor would be advantageous for its largescale usage. To begin, the sensitivity of silver ions at pH 4 and 6 were compared in order to understand if the ligand played a critical role for interacting with silver ions, or if the silver is exclusively forming a sandwich adduct with the AuT. This study was done in the absence of the CS polymer in an effort to minimize any background interference from the polymer and also to obtain a clear picture of the interactions of the trimer with the silver ions. From the data in Figure 4A, a clear relation between the pH and sensitivity is noticed. There was a huge increase in sensitivity of the complex by 2.5 times at pH 6 compared to pH 4. Based on these data, one can conclude that the change in sensitivity is due to deprotonation of the carboxylic acid at pH 6. Therefore, the enhanced sensitivity clearly demonstrates that -- along with the formation of a sandwich structure -- the interaction of free Ag+ ions with the anionic ligand are playing a vital role during the sensing process. Due to this, the rest of the data collected in Figure 4B and 4C were done at pH 6. Figure 4B shows the addition of silver salt followed by addition of KCl salt. Each point on the graph represents a new aliquot of the same stock solution of AuT. The KCl was added after the addition of silver ions to determine if silver precipitation by chloride would affect the PL intensity of the green emission. As seen from the data, KCl addition had no effect on the PL intensity up to about 5 ppm. At higher concentrations of silver ions, the presence of KCl does result in a significant decrease in the PL intensity of the green emission due to the formation of AgCl. We postulate that this decrease in PL intensity occurs after 5ppm because, initially, the sandwich complex is forming, therefore the “sandwiched” silver ions within the cyclic trimer units are unavailable to react with the KCl. However, at higher concentrations of silver ions, the excess silver ions are hypothesized to interact with the carboxylated pyrazolate ligand, based on Figure 4A data, and are freely available to interact with the excess of KCl -resulting in the quenching of the green PL of the adduct. This data set clearly complements the pH-dependent sensitivity of the sensor. Figure 4C shows data from a similar experiment except for the addition of KCl to the trimer before addition of silver salt to determine how KCl would change the sensitivity of the AuT when initially present in the medium. Figure 4C shows clearly that even in the presence of KCl, the complex’s ability to sense silver was not affected, as indicated from the control experiment (black line in Figure 4C). Comparing with the control data, it is evident that the presence of salts like KCl does not affect the complex’s ability to sense free silver ions at low ppm levels. This is a very important result for the application of this sensor in water environments where the water medium is known to contain different salts. These data show that even in the presence of KCl, silver ions seem to preferentially interact with the AuT complex. Therefore, it can be concluded that -- regardless of the order of addition of KCl to AuT -- the sensor herein is still able to remediate silver ions from aqueous solution.

Figure 4: A) AuT silver sensitivity at pH 4 and pH 6 in water. Inset graphic shows the change of the carboxylic acid functional group to carboxylate at pH 4 and pH 6. B) AuT remediation of silver ions in water at pH 6. Silver ions were added to a solution of AuT then excess KCl was added. A fresh solution of AuT was used for every point from the same stock solution. C) AuT remediation of silver ions in water at pH 6. KCl was first added to the AuT then a silver titration was performed. This data set was compared to the control data set (black line) -- which was a silver titration with AuT where no KCl was present. For all PL spectra, λex=320 nm and λem=475 nm. CONCLUSIONS In conclusion, a sensor has been developed that is able to differentiate between Ag+ ions and AgNPs. One of the great utilities of the sensor is to help nanoparticle researchers to 5



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differentiate and understand if the toxicity of the AgNPs is due to AgNPs alone, leaching of silver ions, or due to a combination of both. Currently, differentiation of silver ions from AgNPs directly in solution is not possible using any single existing techniques -- to the best of our knowledge. Differentiation of AgNPs from Ag+ ions is vital since AgNPs are used in many commercially-available consumer products increasing the likelihood of human exposure. Not only was our sensor able to differentiate between Ag+ ions and AgNPs, but it was also able to sense the leakage of Ag+ ions from the AgNPs as well as remediate those ions from solution. The remediation data not only showed the removal/extraction of toxic silver ions from the medium but also demonstrated the first step in making a reusable Ag+ sensor. Further investigations should involve the application of this sensor in biological systems to exactly understand the toxicity mechanism of AgNPs in vitro and in vivo. SUPPORTING INFORMATION None AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (M.A.O.) *E-mail: [email protected] (S.B.M.) † Erin N. Benton and Sreekar B. Marpu have contributed equally to the manuscript.

ACKNOWLEDGMENT M.A.O. acknowledges support to fundamental science aspects of this work to his group by the Welch Foundation (B-1542) and the National Science Foundation (CHE-1413641). We also would like to thank Mr. Donald Benton for his editorial comments. TABLE OF CONTENTS GRAPHICAL ABSTRACT

Ag + Ions Added or Leached

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ACS Applied Materials & Interfaces References (1) Ebabe Elle, R.; Gaillet, S.; Vidé, J.; Romain, C.; Lauret, C.; Rugani, N.; Cristol, J. P.; Rouanet, J. M. Dietary exposure to silver nanoparticles in Sprague–Dawley rats: Effects on oxidative stress and inflammation. Food and Chemical Toxicology 2013, 60, 297301. (2) McShan, D.; Ray, P. C.; Yu, H. Molecular toxicity mechanism of nanosilver. Journal of Food and Drug Analysis 2014, 22, 116127. (3) Petica, A.; Gavriliu, S.; Lungu, M.; Buruntea, N.; Panzaru, C. Colloidal silver solutions with antimicrobial properties. Materials Science and Engineering: B 2008, 152, 22-27. (4) Firooz, A. R.; Ensafi, A. A.; Kazemifard, N.; Khalifeh, R. Development of a highly sensitive and selective optical sensor for determination of ultra-trace amount of silver ions. Sensors and Actuators B: Chemical 2013, 176, 598-604. (5) Zhang, J. F.; Zhou, Y.; Yoon, J.; Kim, J. S. Recent progress in fluorescent and colorimetric chemosensors for detection of precious metal ions (silver, gold and platinum ions). Chem. Soc. Rev. 2011, 40, 3416-3429, DOI: 10.1039/C1CS15028F. (6) Ratte, H. T. Bioaccumulation and Toxicity of Silver Compounds: A Review. Environmental Toxicology and Chemistry 1999, 18, 89-108, DOI: 10.1002/etc.5620180112. (7) Prabhu, S.; Poulose, E. Silver nanoparticles: mechanism of antimicrobial action, synthesis, medical applications, and toxicity effects. Int Nano Lett 2012, 2, 1-10, DOI: 10.1186/2228-5326-2-32. (8) Kittler, S.; Greulich, C.; Diendorf, J.; Köller, M.; Epple, M. Toxicity of Silver Nanoparticles Increases during Storage Because of Slow Dissolution under Release of Silver Ions. Chemistry of Materials 2010, 22, 4548-4554, DOI: 10.1021/cm100023p. (9) Navarro, E.; Piccapietra, F.; Wagner, B.; Marconi, F.; Kaegi, R.; Odzak, N.; Sigg, L.; Behra, R. Toxicity of silver nanoparticles to Chlamydomonas reinhardtii. Environmental science & technology 2008, 42, 8959-8964, DOI: 10.1021/es801785m. (10) Atiyeh, B. S.; Costagliola, M.; Hayek, S. N.; Dibo, S. A. Effect of silver on burn wound infection control and healing: Review of the literature. Burns 2007, 33, 139-148, DOI: 10.1016/j.burns.2006.06.010. (11) Bilberg, K.; Hovgaard, M. B.; Besenbacher, F.; Baatrup, E. In Vivo Toxicity of Silver Nanoparticles and Silver Ions in Zebrafish (Danio rerio). Journal of Toxicology 2012, 2012, 9. (12) Shen, M.; Zhou, X.; Yang, X.; Chao, J.; Liu, J. Exposure Medium: Key in Identifying Free Ag+ as the Exclusive Species of Silver Nanoparticles with Acute Toxicity to Daphnia magna. Open 2015, 5. (13) Liu, L.; Zhang, G.; Xiang, J.; Zhang, D.; Zhu, D. Fluorescence "Turn On" Chemosensors for Ag+ and Hg2+ Based on Tetraphenylethylene Motif Featuring Adenine and Thymine Moieties. Org. Lett. 2008, 10, 4581-4584, DOI: 10.1021/ol801855s. (14) Qin, C.; Wong, W.; Wang, L. A Water-Soluble Organometallic Conjugated Polyelectrolyte for the Direct Colorimetric Detection of Silver Ion in Aqueous Media with High Selectivity and Sensitivity. Macromolecules 2011, 44, 483-489, DOI: 10.1021/ma102373y. (15) Upadhyay, P. K.; Marpu, S. B.; Benton, E. N.; Williams, C. L.; Telang, A.; Omary, M. A. A Phosphorescent Trinuclear Gold(I) Pyrazolate Chemosensor for Silver Ion Detection and Remediation in Aqueous Media. Anal. Chem. 2018, 90, 4999-5006, DOI: 10.1021/acs.analchem.7b04334. (16) Uson, R.; Laguna, A.; Laguna, M.; Briggs, D. A.; Murray, H. H.; Fackler, J. P. In (Tetrahydrothiophene)Gold(I) or Gold(III) Complexes; Kaesz, H. D., Ed.; Inorganic Syntheses; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 1989; 17, pp 85-91. (17) Omary, M. A.; Mohamed, A. A.; Rawashdeh-Omary, M. A.; Fackler, J. P. Photophysics of supramolecular binary stacks consisting of electron-rich trinuclear Au(I) complexes and organic electrophiles. Coordination Chemistry Reviews 2005, 249, 13721381, DOI: 10.1016/j.ccr.2004.12.018. (18) Tekarli, S. M.; Cundari, T. R.; Omary, M. A. Rational Design of Macrometallocyclic Trinuclear Complexes with Superior πAcidity and π-Basicity. Journal of the American Chemical Society 2008, 130, 1669-1675, DOI: 10.1021/ja076527u.


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(19) Yang, C.; Messerschmidt, M.; Coppens, P.; Omary, M. Trinuclear Gold(I) Triazolates: A New Class of Wide-Band Phosphors. Inorganic Chemistry 2006, 45, 6592-6594, DOI: 10.1021/ic060943i. (20) McDougald, J., Roy N.; Chilukuri, B.; Jia, H.; Perez, M. R.; Rabaâ, H.; Wang, X.; Nesterov, V. N.; Cundari, T. R.; Gnade, B. E.; Omary, M. A. Molecular and electronic structure of cyclic trinuclear gold(I) carbeniate complexes: insights for structure/luminescence/conductivity relationships. Inorganic chemistry 2014, 53, 7485-7499, DOI: 10.1021/ic500808q. (21) Omary, M. A.; Rawashdeh-Omary, M. A.; Gonser, M. W. A.; Elbjeirami, O.; Grimes, T.; Cundari, T. R. Metal Effect on the Supramolecular Structure, Photophysics, and Acid−Base Character of Trinuclear Pyrazolato Coinage Metal Complexes. 2005, 44, 8200-8210, DOI: 10.1021/ic0508730.


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