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AgInS2 quantum dots for the detection oftrinitrotolueneTo cite this article: Alfred J Baca et al 2017 Nanotechnology 28 015501

 

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This content was downloaded from IP address 153.90.200.222 on 05/02/2019 at 20:18

AgInS2 quantum dots for the detection oftrinitrotoluene

Alfred J Baca1, Heather A Meylemans1, Lawrence Baldwin1,Lee R Cambrea1, Ji Feng2, Yadong Yin2 and M Joseph Roberts1

1Chemistry Branch Naval Air Warfare Center, Weapons Division (NAWCWD) Research Directorate, CA93555, USA2Department of Chemistry, University of California Riverside, Riverside, CA 92521, USA

E-mail: alfred.baca@navy.mil

Received 15 September 2016, revised 18 October 2016Accepted for publication 26 October 2016Published 28 November 2016

AbstractAgInS2 (AIS) quantum dots (QDs) were synthesized via a thermal decomposition reaction withdodecylamine as the ligand to help stabilize the QDs. This reaction procedure is relatively easyto implement, scalable to large batches (up to hundreds of milligrams of QDs are produced), anda convenient method for the synthesis of chalcogenide QDs. Metal powders of AgNO3 andIn(NO3)3, were used as the metal precursors while diethyldithiocarbamate was used as the sulfursource. The AIS QDs were characterized via transmission electron microscopy, atomic forcemicroscopy, and energy dispersive x-ray spectroscopy. As an application for these less toxicnanomaterials, we demonstrate the selective detection of Trinitrotoluene (TNT) at concentrationsas low as 6 micromolar (μM) and without the functionalization of a ligand that is specificallydesigned to interact with TNT molecules. We also demonstrate a simple approach to patterningthe AIS QDs onto filter paper, for the detection of TNT molecules by eye. Collectively, the easeof the synthesis of the less toxic AIS QDs, and the ability to detect TNT molecules by eyesuggest an attractive route to highly sensitive and portable substrates for environmentalmonitoring, chemical warfare agent detection, and other applications.

S Online supplementary data available from stacks.iop.org/NANO/28/015501/mmedia

Keywords: quantum dots, environmental monitoring, nanotechnology, energetic materials,nanoparticles

(Some figures may appear in colour only in the online journal)

1. Introduction

Trinitrotoluene (TNT) has various applications in industrialsettings, and is also used as energetic material in militaryordnance. Of highest concern to national security, are appli-cations where TNT is used to form make-shift bombs andwhen used in terrorist activities. A second area of concern forTNT is in the area of environmental monitoring [1]. Militaryinstallations contaminated with TNT are a major concern forthe Department of Defense. TNT can be found in waterstreams and soils near these military installations, thereforeTNT presents an environmental concern and possible healthhazard to those exposed to trace amounts of TNT. Traditionalmethods used for TNT sensing (in soil samples) are limited in

their analytical capability, generally slow and non-portable.As a result these methods are not practical in scenarios thatrequire fast and sensitive detection of TNT.

Aside from traditional methods for detection of TNT,there exist approaches that rely on investigating the materialschemistry and properties for the sensing of TNT [2]. Due totremendous progress in the area of synthesis of low dimen-sional materials there has been a shift in the paradigm forTNT detection. TNT sensing has been demonstrated with lowdimensional materials in forms of nanoparticles [3] andquantum dots (QDs). In the case of nanoparticles, most of thedetection has been demonstrated by exploiting the inherentplasmonic properties of noble metal nanomaterials. Quantumdot sensing of TNT relies on the fluorescence emission

Nanotechnology

Nanotechnology 28 (2017) 015501 (8pp) doi:10.1088/0957-4484/28/1/015501

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modulation associated with interaction between the QDs andthe TNT molecules. For example, Cai et al demonstrated asimple and selective method to detect TNT by using CdTeQDs functionalized with L-Cysteine as a ligand. In this work,a Meisenheimer complex between the TNT and the aminoacid ligand was formed and resulted in a detection schemethat can be used to determine TNT at concentrations as low as1.1 nM by monitoring the fluorescence quenching uponexposure to the functionalized QDs [4]. Soundararajan et alrecently demonstrated a related approach where CdTe andcore–shell CdTe–ZnS QDs functionalized with methioninewere used to detect nitroaromatic explosive materials [5]. Theadvantage of this work is that the QDs are water soluble anddemonstrate a high quenching constant for the variousexplosive chemicals used. Although great progress has beendemonstrated in these two research reports and others, the useof QDs synthesized from toxic metals, such as Cd, limits theirapplication in real world demonstrations due to environmentalconcerns with the use of certain heavy metals. An alternativeapproach is to use QDs formed from less toxic materials suchas those recently demonstrated in the literature. For example,Zhang et al recently reported highly luminescent MoOx QDsfor determination of TNT with good sensitivity without theuse of any sample pretreatments prior to analysis [6]. Nano-materials composed of less toxic materials represent a newand more environmentally friendly approach to the detectionof explosive chemicals [2, 6–11]. Our group [12] and others[13–16] have recently explored the synthesis and applicationsof less toxic QDs composed of more environmentally friendlymetals. To this end, we recently demonstrated the applicationof zinc silver indium sulfide (ZAIS) and related QDs for theapplication of metal detection [12]. In this report, we show thesynthesis of AgInS2 based QDs and the application of theseQDs as fluorometric probes for the determination of TNT,without the use of ligands specific to TNT. We demonstratethe sensitivity and selectivity of the AIS QDs when exposedto non-aromatic and aromatic explosive chemicals. Further-more, we demonstrate the detection of TNT molecules by eyeand present a simple patterning method for AIS QDs on lowcost filter paper substrates. To the best of our knowledge, thispaper is the first report of the application of AIS QDs for thedetection of TNT and other organic chemicals and is a newresearch direction for these particular chalcogenide nanoma-terials. Collectively the ease of fabrication, use of less toxicmaterials, and the selectivity to nitroaromatic explosive che-micals results in a practical solution to the development of aportable fluorescent probe based on AIS QDs for the deter-mination of TNT.

2. Experimental details

2.1. Materials and instruments

All reagents and materials were used as received. The sol-vents, ligand dodecylamine, and diethyldithiocarbamate, werepurchased from Sigma-Aldrich. Metal powders of AgNO3

and In(NO3)3 (purity 99.999%) used in the quantum dot

synthesis were purchased from Strem Chemical. SU-8 pho-toresist and developer were purchased from Microchem.TNT, CL-20, and RDX were obtained in-house. Solutions ofeach material were dissolved in reagent grade acetone.Scanning electron microscopy (SEM) and energy dispersivespectroscopy (EDS) spectra of the samples were taken at20 kV accelerating voltage on a Zeiss EVO50-EP SEM withEDAX system. Atomic force microscopy (AFM) images weretaken with Asylum MFP-3D. Absorption spectra were mea-sured with a VWR 1600PC UV–vis spectrophotometer. Themorphology of the QDs was examined by using Tecnai 12transmission electron microscope at an accelerating voltage of120 kV. Images were recorded by using a Gatan US1000digital CCD camera. Emission spectra were collected from420 to 780 nm using an excitation wavelength of 395 nm on aHoriba Jobin-Yvon FluoroMax-4 fluorimeter. An excitationslit width of 1 nm and emission slit width of 2 nm was usedfor solution fluorimeter scans. The noise present in thefluorimeter scans are a result of a constant slit width settingused throughout the experiments presented herein (please seesupporting information figure S1). For patterned solid stateQD TNT detection (on filter paper), excitation and emissionslit widths of 2 and 5 nm were used, respectively.

2.2. Synthesis of AgInS (AIS) QDs

AIS QDs were synthesized using a modified approach to ourpreviously published work [12]. Briefly, a 1:1 molar ratio ofAgNO3 and In(NO3)3 were combined with 2 molar equiva-lents of diethyldithiocarbamate and stirred vigorously in waterfor 15 min. The powder was then filtered and dried undervacuum overnight. The powder was then placed in a 50 mlround bottom flask heated for ∼30 min under a nitrogenenvironment with vigorous stirring without solvent. Anexcess amount of dodecylamine (10 ml) was added quicklyusing an 18 gauge needle and a 12 ml plastic syringe to theheated solution and after 3 min the solution was removedfrom the heat and immediately centrifuged. After addition ofthe 10 ml of dodecylamine, care must be exercised to ensurethat the resulting solution is only heated for 3 min to improvethe synthesis of AIS QDs using this thermal decompositionmethod. This is a key step and variations in this step canresult in AIS QDs with a slightly different spectral position ofthe max emission peak. For this report we used and collectdata from the same batch of as produced AIS QDs. The AISQDs were then isolated by centrifugation with methanol. Foruse in this investigation, the QDs were dispersed in chloro-form at a concentration of 1 mg ml−1.

2.3. Detection of energetics with AIS QDs

Different concentrations composed of TNT, CL-20, and RDXwere prepared by weighing out the appropriate amount ofsolid energetic materials in a glass vial and the solid materialswere dissolved in acetone. Acetone was used as a solvent dueto the higher solubility of the non-aromatic energetic mate-rials, and acetone (without energetic) was added to each AISQD solution to maintain the dilution factor. For the energetic

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Nanotechnology 28 (2017) 015501 A J Baca et al

analysis a glass vial was filled with 1 ml of AIS QD solutionfollowed by the addition of 2 ml of the energetic chemicals.This was done for all of the solutions prepared for this workand either acetone or energetic material (dissolved in acetone)was added to maintain a 1:2 ratio by volume of AIS QD toenergetic. Each emission spectrum is representative data fromat least three different sample runs and all data collected werefrom the same batch of AIS QDs. Emission spectra werecollected after 1 h of incubation after adding the two solu-tions, which were left undisturbed during this period. Shorterincubation times were (∼1 min) examined and data werecollected, however, 1 h incubation while leaving the sampleundisturbed yielded the best results. However, we would liketo note that a visible change were noticed within a few sec-onds for TNT sensing on paper substrates, and as a result webelieve AIS nanomaterials can be used as a real time, fastresponse sensor test bed for TNT determination.

2.4. Patterned AIS QDs on paper substrates

To demonstrate the detection of TNT molecules by eye, AISQDs were patterned onto filter paper (Whatman #1) by usingconventional photolithographic processing of a negativephotoresist (Microchem SU-8). Briefly, filter papers weresoaked in SU-8 #5 and allowed to prebake at 95 °C until dry.A broadband UV source (OAI) through a chrome mask wasused to pattern the SU-8 onto the filter papers followed by apost bake at 95 °C for 3 min and developed in SU-8 developerfor ∼3 min. The as-prepared patterns were subsequentlysoaked in a 1 mgml−1 AIS QD solution in chloroform for2 min, removed from the QD solution, and then heated for1 min at 90 °C in order to help with the solvent removal. Dueto the solvophobic nature of SU-8 photoresist, and the pre-sence of the photoresist through the thickness of the filterpaper, the QD’s deposited into the clear sections of the filterpaper. Due to the high surface area of the fibers in the paper, arelatively high concentration was deposited. By this method,the patterning of the AIS QDs onto low cost paper substrateswas achieved. For the cotton swab demonstration, a cottonswab was soaked in a 1 mg ml−1 solution (∼10 ml) of AISQDs and allowed to dry prior to analysis.

3. Results and discussion

The most significant advantage and novelty of this manu-script, is the relatively lower toxicity of AIS QDs in com-parison to those nanomaterials based on Cd and or Pb. Therehave been several reports that detail the reduced toxicity ofZnAgInS2 (ZAIS), CuInS2/ZnS, and ZnSe QDs for biome-dical research and imaging studies [14, 17, 18]. Recentstudies have reported, via cytotoxicity assays, that ZAISQDs showed a significantly improved biocompatibility incomparison to the CdSe/ZnS QDs [14]. Further studies willbe needed to address the long-term toxicity effects of AISQDs, but based on these results we believe our cadmium freeAIS QDs to be a less toxic than cadmium based nanoma-terials and represent a possible alternative to nanomaterials

that use heavy metals with known negative environmentaleffects [19]. For the synthesis of AIS QDs, metal powders ofAgNO3 and In(NO3)3 were used in a thermal decompositionreaction while using dodecylamine as the ligand to generatefluorescent AIS QDs. By adding a stirring step, the synthesiscould be scaled up and it improved the QD yield. Synthesiswith this method yielded quantities of QDs as high as500 mg per batch. We found these particular QDs to bedispersible in various solvents which include acetonitrile,chloroform, toluene, chlorobenzene, anisole, and dichlor-obenzene, however, chloroform was the best solvent as theQDs did not aggregate over time, and led to highly lumi-nescent quantum dot solutions as shown in the optical imagein the inset of figure 1. Figure 1(a) shows the optical prop-erties of the as prepared QDs. The black curve shows thefluorescence emission of a 1 mg ml−1 solution of AIS QDsin chloroform. The maximum emission was centered at600 nm at an excitation wavelength of 395 nm as shown infigure 1. The broad, strong emission peak is typical of thesechalcogenide QD nanomaterials [12, 14–16]. The FWHMfor AIS QDs synthesized via our thermal decompositionroute is 111 nm. The red spectrum in figure 1 depicts theUV–vis of solution of AIS QDs. The AIS QDs showed agood absorption in the UV and blue wavelengths.Figure 1(b) shows a representative SEM image of a thin filmof AIS QDs. For the fabrication of thin films of AIS QDs a 1inch by 1 inch silicon wafer with an orientation of (100) wascut and cleaved. The wafer chiplet was vigorously rinsed inacetone, isopropyl alcohol, and deionized water, respec-tively. The wafer was dried with a stream of nitrogen and toform a thin film of AIS QDs, a 1 mg ml−1 solution of AISQDs in chloroform was dropped cast and allowed to dry.Several layers were added (seven rounds of ∼1 ml dispensedvolume) on top of the dried AIS layer were performed untilan average thickness of ∼100 nm was obtained as deter-mined by a stylus profilometer. The morphology of AIS QDsin a thin film format (100–138 nm thick) shows a flower-like, rough layout of aggregated QDs. In order to verify thepresence of the metal and sulfur sources in our AIS QDs, wealso analyzed thin films of AIS QDs with EDAX.Figure 1(c) shows EDAX spectra of the AIS QD thin film of∼100 nm thick shown in figure 1(b). The Ag, In, and Selemental analysis peaks are denoted in the spectra andconfirms presence of these elements in our as preparednanomaterials.

TEM and AFM confirmed the presence of sphericalquantum dot materials. Figure 2 shows the TEM and AFMimages of a representative QD batch synthesized in this work.TEM images revealed small particle distributions with anaverage size of ∼1.5 nm as show in figure 2(a). In order todetermine the height of the AIS QDs we measured the AFMline profile of a dilute solution on AIS QDs dispensed on asilicon wafer. Figure 2(b) shows a 3.0 μm×3.0 μm AFMscan of the AIS QDs. A line profile was used to determine theheight of the smaller QD clusters shown in the AFM image infigure 2(b) as shown in the black dotted line in figure 2(b).According to the AFM software the AIS QDs have a height of1.2–1.4 nm as depicted in figure 2(c). The AIS QDs aggregate

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Nanotechnology 28 (2017) 015501 A J Baca et al

when the solvent is dried (figure 2(b)) and we hypothesis thatdodecylamine keeps the QD cores from direct contact tominimize self-quenching. However, for our application in thispaper (i.e. sensing TNT) the aggregation of the AIS nano-materials does not directly affect our proposed sensingmechanism.

CdS and other nanomaterials have been shown in pre-vious reports to work as sensing platforms for energeticmaterials [4]. Some published reports propose an electrontransfer mechanism from the QD metal center to the electron

Figure 1. (a) Shows the optical properties of AIS QDs. The emission(black curve) and absorption (red curve) spectra are shown. Insetshows an optical image of a representative AIS QD solution inchloroform. Figure 1(b) shows a representative SEM image of a thinfilm of AIS QDs on a silicon wafer substrate and figure 1(c) showsthe EDS spectroscopy of the thin film composed of aggregatedAIS QDs.

Figure 2. Shows the morphology and size distribution of the AISQDs. Figure 2(a) shows a TEM image of AIS QDs, figure 2(b)shows an AFM image of a dilute solution of AIS QDs dispensed ona silicon wafer and figure 2(c) shows the AFM height profile fromthe image in 2(b) that was obtained from the same AFM plot (blackdotted line in figure 2(b)).

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deficient aromatic ring of the TNT molecule [9, 20]. Otherreports attribute the quenching of the QD fluorescence to theformation of a Meisenheimer complex between the TNT andligands on the surface of the QDs [4–6, 10, 21, 22]. In thisreport, we used dodecylamine as a ligand to minimize theinteraction between the TNT molecules and the ligand, unlikeother reports where the ligand acts as a binding site for TNTmolecules. We chose dodecylamine to maintain the aminoend towards the quantum dot center. We performed a controlexperiment where we exposed a few crystals of TNT to asolution containing dodecylamine and acetone and a solutioncontaining dodecylamine and chloroform both with no AISQDs. We found that an amine-TNT complex (via donor-acceptor interaction) is formed when the TNT is added to theacetone solution, and observed no color change for thesolution with chloroform as the solvent. Please see supportinginformation figure S2. This is in good agreement with workdone by Latenresse et al who established the role of thesolvent and water and its effects on the interaction of TNTwith hydroxide and amines [23, 24]. We presume that theamine-TNT complex that is formed in our case, leads to thequenching observed in the following results and discussion.When TNT (in acetone) is added to the dodecylamine func-tionalized AIS QDs in chloroform it results in quenching ofthe QDs due to the presence of amine-TNT complexes. Thecomplex formed by amines and polynitroaromatics (TNT andDNT) in various solvents has been established by previousreports [25, 26]. To this end, in order to study the interactionof AIS QDs with TNT-amine complexes, we studied theinteraction of TNT at different concentrations in acetonewhile the AIS QDs were dissolved in chloroform. Figure 3(a)shows the AIS quantum dot fluorescence response with a 1:2ratio, by volume, of QD to energetic solution. The topgraph (black curve) shows the fluorescence emission of AISQD (1 mg ml−1 in chloroform) with the addition of acetonewith no TNT(∼2 mls to keep dilution factor), the secondgraph (red curve) depicts the emission of AIS QDs with theaddition of a 1 mM TNT solution, the third graph (greenspectra) shows the emission of AIS QDs with the addition ofa 5 mM TNT solution and the bottom graph (blue spectra)shows the emission of AIS QDs with the addition of 10 mMTNT solution. Acetone was used as a solvent due to thehigher solubility of the non-aromatic energetic chemicals inacetone, versus chloroform. Acetone was used as a blankthroughout the work and the effect of acetone and chloroformon the emission of AIS QDs is shown in the supportinginformation (please see figure S3). Figure 3(a) spectra showthe quenching of the emission response of the AIS QDs withthe subsequent addition of TNT at various concentrationswhich can be partially attributed to the proposed quenchingmechanism resulting from aggregation of the AIS QDs whenusing acetone as a solvent in the presence of dodecylamine.The inset in figure 3(a) shows an optical image of a solutionof AIS QDs with no TNT, 1 mM TNT, 5 mM TNT, and10 mM TNT ( from left to right). In order to investigate thelimit of detection of AIS QDs with dodecylamine as a ligand,we also studied lower TNT concentrations to investigate theirinteraction with the AIS QDs. Figure 3(b) shows the emission

response of AIS QDs for various TNT concentrations. Wewere able to detect TNT in acetone at concentrations as low as6 μM. The monitored fluorescence response is from a 1:2ratio, per volume, of QD to energetic solution. We would liketo note that controlling the heating step during the addition ofdodecylamine can yield QDs of possibly slightly differentdiameter or composition based on observed shifts in the maxemission. A lower detection limit may be possible bydesigning AIS QDs with a different ligand that can selectivelybind to the TNT analytes, increasing the spectrometer slitwidth, and by lowering the concentration of QDs used for ourmeasurements. The goal of this work is to introduce a processthat can be used to develop a colorimetric system and there-fore, concentrations lower than micromolar range are harderto visually detect and will require instrumentation due to therelatively low quantum yield of these nanomaterials [12].Methods to create aggregated materials in solution using these

Figure 3. (a) Shows the emission of a AIS QD solution with no TNT(top graph; black curve), AIS QD solution with 1 mM TNT ( redcurve), AIS QD solution with 5 mM TNT ( green curve), and AISQD solution with 10 mM TNT ( blue curve). Inset shows an opticalimage of AIS QDs exposed to TNT at 1, 5, and 10 mM (from left toright). Figure 3(b) Shows the emission of a AIS QD solution with noTNT ( top graph; black curve), AIS QD solution with 4 μM TNT (red curve), AIS QD solution with 6 μM TNT ( green curve), and AISQD solution with 8 μM TNT ( blue curve), AIS QD solution with10 μM TNT ( orange curve).

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materials can also be pursued in future work [27]. For thisparticular work we are interested in detection levels that areeasily visible by the naked eye, as we demonstrate in thefollowing section using low cost filter paper substratesimmobilized and patterned with AIS QDs. Ligand variationand a detailed study of the detection of TNT in water or soilusing AIS QDs are topics of future research.

In order to show the selectivity of our AIS QDs towardsTNT, we investigated the quenching of the QDs whenexposed to various other energetic chemicals. Figure 4(a)depicts the emission spectra of AIS QDs when exposed to noenergetic (black spectra), 10 mM RDX (red spectra), 10 mMCL-20 (green spectra), and 10 mM TNT (blue spectra). TheAIS QD fluorescence response is from a 1:2 ratio, pervolume, of QD to energetic solution. Figure 4(a) shows theselectivity of AIS QDs depicting the small changes in theemission intensity for 10 mM RDX and 10 mM CL-20 incomparison to the emission of 10 mM TNT solution. Tofurther demonstrate the selectivity of AIS QDs towardsnitroaromatic chemicals, figure 4(b) shows the selectivity plot

of AIS QDs when exposed to TNT, RDX, and CL-20 whereIo corresponds to the max emission intensity at 600 nm forAIS QDs with no energetic and I corresponds to the maxemission intensity at 600 nm for the different energetic che-micals. In this example the TNT interacts with the AIS QDmetal core to quench the emission. RDX and CL-20 mole-cules have some interaction with the QD core, but to a lesserextent.

Issues with solvent compatibility with solution basedsensing can be avoiding by transfer the nanomaterials into asolid state test strip type device. For example, our AIS QDsare suspendable in chloroform, but real world applicationswill require TNT sensing in soil or water both which are notcompatible with chloroform. The solvent effects are drivingus to look at making solid test samples. The overall goal is toremove solvents from the system so that detection can bedone on a wider variety of samples. To this end, we havedeveloped a solid state test sensor using the AIS QDs as afluorometric probe without solvent on low cost filter papersubstrates and cotton swabs. A key advantage of AIS QDs istheir low toxicity when compared to other heavy metal basedchalcogenide QDs. As a result, developing a portable QDsensor on a low cost substrate is of great interest and couldhave environmental applications. As a result, we developed asimple patterning process that can be used with variousnanomaterials and when combined with microfluidics canlead to a low cost, low sample consumption portable fieldsensor for TNT detection. We also demonstrate the versatilityof AIS QDs as these nanomaterials can be used in solutionand as well as in a solid form for portable detection andscreening of TNT. To demonstrate this application, inspiredby work done by Professor Whitesides group [28, 29] andothers [30], we soaked filter paper with photoresist and usedphotolithography to pattern the photoresist onto the surface ofthe filter papers. To test the optical emission and stability ofthe patterned sample (i.e. dry AIS QDs) we placed the pat-terned substrate in our fluorimeter and measured the emissionbefore and after exposure to TNT. Figure 5(a) shows theemission response of the patterned QDs in the solid state; AISQDs were solution cast onto a patterned filter paper (solidline; black spectra) and the emission response (red spectra)was collected after placing a few drops of 5 mM TNT onto thepatterned areas. The dotted line black spectrum shows theemission response of photoresist on a filter paper in theabsence of AIS QDs and the dashed line black spectrumshows the emission of the patterned area without photoresistand AIS QDs. The line spectrum shows a large backgrounddue to the auto fluorescence and optical light scattering of theSU-8 coated filter paper. There is a small red shift in the maxemission for filter paper based AIS QDs (∼2 nm) and FWHMfor the layout is 130 nm. An important analytic consequenceof this patterning method is that it allows for the detection ofTNT and possibly other analytes by eye. Optical images ofpatterned AIS QDs on paper substrates before and afterexposure to 10 mM TNT are shown in optical image on theleft of figure 5(b). The AIS QDs were patterned into sevendiscrete line patterns that are 1 mm wide and 15 mm long. Asdepicted in optical image on the right in figure 5(b), the

Figure 4. (a) Shows the emission intensity of AIS QD solutionexposed to 10 mM energetic solutions of RDX, CL-20, and TNT.Figure 4(b) shows a column graph showing the selectivity of the AISQDs. I and Io represent the max emission at 600 nm for the energeticand AIS QD solution respectively.

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Nanotechnology 28 (2017) 015501 A J Baca et al

quenching of three lines of AIS QDs is observed uponexposure to 10 mM TNT. AIS QDs can also be placed ontocotton swabs for a portable platform to TNT sensing, and arelated example would require a portable substrate that can beused in a test kit for analysis and for the determination of

TNT in soil and water for real world applications. To this endwe demonstrate the ability of AIS QDs to be suspended in acotton swab fiber for such analysis. Figure 5(c) shows acotton swab with dry AIS QDs before and after exposure to1 mM TNT solution. Visible changes were noticed within afew seconds of exposure to the TNT for both filter and cottonbased sensors. As a result, we believe these nanomaterials canbe used in a portable test kit for TNT detection. Collectively,these results demonstrate a test bed device and colormetricsensing of these less toxic nanomaterials that could lead to thedevelopment of a portable low cost TNT optical sensor.

4. Conclusion

Here we presented a simple approach to the synthesis of AISQDs via a thermal decomposition reaction. The AIS QDsshowed strong fluorescent properties that can be tuned todetect TNT molecules at concentrations as low as 6 μM insolution. The AIS QD nanomaterials can be patterned on lowcost paper substrates for portable TNT field detection by eye.Collectively, the ease of synthesis, ability to pattern thenanomaterials, and the ability to control and tune the opticalproperties of the QDs suggest an environmentally friendlyroute to the development of a low cost sensor for portablewarfare agent detection, environmental remediation, energeticsensing, and others.

Acknowledgments

This work was supported by NISE project managed by DrSteve Fallis and Mallory Boyd. AJB would like to thank DanConnor for SEM imaging and EDS analysis. AJB would likethank Professor John A Rogers and the Rogers group forlending a copy of the photomask used in this work.

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Figure 5. (a) Shows the emission of AIS QDs on paper substrates(black curve) and after exposure to 10 mM TNT (red curve). Thedotted line in figure 5(a) shows the emission of an SU-8 coated papersubstrate with no exposure to AIS QD or energetic solution while thedashed line shows the emission of the patterned area without SU-8 orAIS QDs. Figure 5(b) shows optical images of SU-8 patterned papersubstrates with infiltrated AIS QDs in the non-patterned regions (leftimage) and after exposure to 5 mM TNT (right image). Scale bar is15 mm. The photo-lithographically patterned lines are 1 mm wideand 15 mm long. Figure 5(c) shows an optical image of a cottonswab with AIS QDs (top cotton swab) and after exposure to 1 mMTNT (bottom cotton swab). Scale bar is 10 mm.

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