Highly selective and sensitive detection of Cr6+ ions using size-specific label-free gold nanoparticles

Accepted Manuscript Title: Highly selective and sensitive detection of Cr6+ ions using size-specific label-free gold nanoparticles Author: Rajalakshmi Kanagaraj Yun-Sik Nam Sung Jin Pai Sang Soo Han Kang-Bong Lee PII: DOI: Reference:

S0925-4005(17)30912-7 http://dx.doi.org/doi:10.1016/j.snb.2017.05.089 SNB 22373

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

1-2-2017 17-5-2017 17-5-2017

Please cite this article as: R. Kanagaraj, Y.-S. Nam, S.J. Pai, S.S. Han, K.B. Lee, Highly selective and sensitive detection of Cr6+ ions using sizespecific label-free gold nanoparticles, Sensors and Actuators B: Chemical (2017), http://dx.doi.org/10.1016/j.snb.2017.05.089 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Rajalakshmi Kanagaraja, , Yun-Sik Namb, ,

Green City Technology Institute, bAdvanced Analysis Center, cComputational Science Center,

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Sung Jin Paic, Sang Soo Hanc, Kang-Bong Leea,*

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Highly selective and sensitive detection of Cr6+ ions using size-specific labelfree gold nanoparticles

Korea Institute of Science & Technology, Hwarang-ro 14-gil 5 SeongBuk-gu,

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Seoul 02792, Republic of Korea.

Corresponding author. Tel.: +82 2 958 5957; fax.: +82 2 958 5810 E-mail address: [email protected] (K.-B. Lee).

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These authors contributed equally to this work.

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ABSTRACT Gold nanoparticles (AuNPs) of various sizes were synthesized by modifying the citrate concentration; 45-nm AuNPs were found to respond to Cr6+ ions selectively. 45-nm, label-free AuNPs showed localized surface plasmon resonance bands at 530 nm, which decreased linearly upon addition of Cr6+. The addition of Cr6+ also resulted in the appearance of a new band at 750 nm, along with a visible color change in the solution from wine

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red to violet. The decrease in absorbance and color change were due to AuNP aggregation upon coordination of Cr6+. The detection limit for Cr6+ was 0.4 nM, and excellent selectivity was observed in the presence of other

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metal ions and anions. The binding site and sensing mechanism for Cr6+ and label-free AuNPs were

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characterized by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. This method was applied to tap, pond, and waste water samples, and validated using inductively coupled plasmaoptical emission spectrometry, illustrating the utility of the sensitive, selective, and simple AuNP sensor in the

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detection of Cr6+.

Keywords: Label-free AuNPs, Colorimetric AuNPs sensor, Size-specific nanoparticles, Selective Cr6+ detection,

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DFT calculation

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1. Introduction

Chromium ions are present in two oxidation states in aqueous solutions: Cr3+ and Cr6+. Cr3+ ions have important roles in carbohydrate metabolism, enzyme activation and nucleic acid stabilization [1]. In contrast, Cr6+ is known to be highly carcinogenic and toxic to humans. Cr6+ induces a variety of clinical problems, such as DNA

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damage, oxidative stress, gene alteration, and apoptotic cell death. Chromium ions are widely used in a number of commercial applications, such as chrome plating, pigment production, leather tanning, dying, sanitary landfill

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leaching, and stainless processing [2]. Cr6+ ions are ubiquitous in the environment, and can even be detected in natural water because of their large-scale use in various industries. The U.S. Environmental Protection Agency

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and World Health Organization have established total allowable chromium contents of 0.1 mg/L and 0.05 mg/L, respectively, in drinking water. Cr6+ and Cr3+ are included in the total chromium content because they can

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interconvert in water and in the human body, depending on the environmental condition.

Therefore, monitoring Cr6+ ions in the environment is essential [3]. Several methods are available for Cr6+

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detection, including inductively coupled plasma-mass spectrometry (ICP-MS) [4], voltammetry [5], X-ray fluorescence [6], and fluorescence [7]. However, these methods require tedious sample pretreatment procedures,

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complicated instruments, and highly trained operators. Therefore, the development of a simple, rapid, and

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convenient analytical technique for the selective detection of Cr6+ ions remains a challenge. The selective monitoring of Cr6+ and Cr3+ ions in aqueous solution is very difficult because Cr6+ is easily reduced to Cr3+ during sample pretreatment [8]. Therefore, a facile and selective assay method for Cr6+ ions, irrespective of the presence of Cr3+, is required.

Various novel nanotechnology-based colorimetric assays have been developed for the analysis of Cr6+ ion, including those based on gold nanoparticles (AuNPs) functionalized with 1,4-dithiothreitol [9], nanorods [10], label-free silver nanoparticles (AgNPs) [11], Agcore−Aushell NPs [12], AgNPs conjugated with leaf extract [13], Tween 20-stabilized AuNPs [14], AuNPs modified with 1,5-diphenylcarbazide [15], and AgNPs capped with tartaric acid [16]. However, analytical methods using modified nanoparticles have shown limited selectivity and sensitivity in the detection of Cr6+ ions. It is well-established that the electronic properties of nanoparticles are size-dependent. Intrinsic size changes in AuNPs were found to modulate their physical and chemical properties, and such effects were related to surface charge density [17,18]. Variation in the optical properties of AuNPs as a function of size were found to be useful for the determination of various metal cations and anions, and 45 nm AuNPs reacted with Cr6+ ions

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selectively [19]. We report a highly sensitive and selective monitoring method for Cr6+ ions in aqueous solutions containing various metal ions and anions using AuNPs. The sizes of the AuNPs were adjusted by simply varying the concentration of sodium citrate in the citrate reduction. Nanoparticles are generally capped with citrate or borohydride, since these anions are used as reducing agents. These anion-capped nanoparticles in this study

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were referred to as “label-free” nanoparticles. Label-free nanoparticles are usually conjugated with specific ligands for applications. Therefore, in this study, "label-free" nanoparticles means ligand-free nanoparticles.

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Dispersed, and aggregated AuNPs were characterized by ultraviolet-visible spectroscopy (UV-Vis), high resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD) spectroscopy, zeta potential,

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and dynamic light scattering (DLS) upon addition of Cr6+. Cr6+ ion binding sites on the surface of label-free AuNPs were elucidated by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass

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spectrometry (TOF-SIMS). Furthermore, the interaction energy between Cr6+ (or Cr3+) and citrate on the surface of the AuNPs was calculated using density functional theory (DFT). The stability of the Cr6+-citrate complexes

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suggested a selective affinity towards Cr6+ over Cr3+ by the label-free AuNPs.

This proposed assay using AuNPs was very simple, cost-efficient, and allowed for the on-site detection of

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Cr6+ in real time. The limit of detection (LOD) was 0.4 nM by UV-Vis and 4 nM using the naked eye.

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Therefore, this technique could be utilized to monitor Cr6+ ions in a wide range of practical samples.

2. Materials and Methods

2.1. Chemicals and reagents

Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4⋅3H2O) and trisodium citrate (TSC) were purchased from Sigma-Aldrich. The salts of metal ions (Cr6+, Cr3+, Co3+, Al3+, Mg2+, Fe3+, Li+, Ba2+, As3+, Cu2+, Ni2+, Ca2+, Na+, Ga3+, Cd2+, Ge4+, Mn2+, and Sn2+) and anions (NO2−, ClO4−, NO3−, SO42−, PO43−, F−, Cl−, Br−, I−, CH3CO2−, C3H5O(COO)33−, and C6H5(COO) −) were obtained from AccuStandard (New Haven, CT, USA). HCl and NaOH were obtained from Samchun chemical (Gyeong Gi-Do, Republic of Korea). Double distilled water was prepared using a Milli-Q water purification system from Millipore (Bedford, MA, USA) and used throughout this study. Reagents were used as received without further purification. To test the utility of the method, tap,

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pond, and waste water samples were collected from the Korea Institute of Science and Technology (KIST) campus.

2.2. Instrumentation

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UV-Vis absorption spectra were measured in the range of 300−800 nm, using polystyrene cells with a 1-mm path length on a Spectrophotometer S-3100 (Sinco, Seoul, Republic of Korea). Dynamic light scattering (DLS)

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measurements were conducted using a Zetasizer (Malvern Instruments Ltd, Worcestershire, UK). Zeta potentials was measured using the same Zetasizer. Images and diameters of the label-free AuNPs and aggregated Cr6+-

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AuNPs were obtained using TEM (Titan, FEITM, Oregon, USA). XPS was performed using a PHI 5000 VersaProbe (Ulvac-PHI, Kanagawa, Japan) with a background pressure of 2.0×10−7 Pa, monochromator Al Kα

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source (1486.6 eV) and anode (24.5 W, 15 kV). XRD analysis was performed on a D/max-2500PC (Rigaku, Tokyo, Japan). Mass spectra were measured using TOF-SIMS (IONTOF, TOF-SIMS 5, Münster, Germany).

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Solution pH was measured using an HI 2210 pH meter (Hanna instruments, Woonsocket, RI, USA). Cr6+ concentrations in real samples were confirmed by inductively coupled plasma-optical emission spectrometry

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(ICP-OES) (iCAPTM 6000, Thermo Fisher Scientific, Waltham, Massachusetts, USA).

2.3. Computational methods

To investigate atomic level coordination and energetics, DFT calculations of model systems were carried. As the AuNPs were too big for ab initio calculations, we assumed that they made no apparent contribution to chromium ion binding. The model system consisted of citrate molecules with hydrogen terminated at the AuNP sites and Cr3+ ions. The solvent (water) was implicitly considered as a dielectric medium. The Q-Chem 4.3 program was used for quantum calculations [20]. The DFT calculation level was the Becke three-parameter plus the Lee−Yang−Parr (B3LYP) functional [21,22] and the Pople 6-31++G** basis set [23]. Solvent effects were accounted for using a conductor-like polarizable continuum model with a dielectric constant of 78.4 [24,25].

2.4. Preparation of size-controlled AuNPs

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The AuNPs were prepared by simple reduction of HAuCl4 with TSC as reported elsewhere [26,27]. A 0.3mM solution of HAuCl4 was prepared in a round-bottom flask with water (100 mL). The solution was heated with continuous stirring in a reflux condenser. Upon boiling, a 38.8-mM TSC solution was slowly injected. The color of the solution changed from pale yellow to wine red. The solution was heated at reflux for 30 min. Next, the solution was cooled to room temperature and stored in the refrigerator at 4 °C. AuNPs of various sizes were

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prepared using TSC concentrations of 2.5, 2.2, 1.8 and 1.4 mM, respectively [26,27].

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prepared by changing the concentration of TSC in the above method. 30, 45, 55 and 70 nm AuNPs were

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2.5. Preparation of real samples

Real water samples were collected from the laboratory tap and a pond at KIST. Tap water samples were

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used without further purification and spiked with a certain concentration of Cr6+ ions. Pond and waste water samples were filtered through 0.45- and 0.2-µm membranes and then spiked with the standard solution of Cr6+

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ions.

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2.6. General procedure for the colorimetric sensing of Cr6+ions

In order to determine Cr6+ concentrations using label-free AuNPs, a range of Cr6+ concentrations were prepared (0.1-1.0 ppm). Each Cr6+ solution was added to 1.8 mL of the AuNP solution, and the resulting mixtures were allowed to react for 70 min. Subsequently, changes in absorbance were monitored using UV-vis. All of the experiments were carried out at room temperature. The determination of Cr6+ was accomplished by plotting A750/A530 vs. the concentration of Cr6+.

3. Results and discussion

3.1. Characterization of label-free AuNPs, and selective sensing of Cr6+

AuNPs of various sizes were synthesized by changing the concentration of sodium citrate. The resulting AuNPs were tested as sensors for Cr6+. The selective monitoring of Cr6+ in the presence of Cr3+ is a challenge because Cr3+ is easily oxidized to Cr6+ during sample pretreatment for instrumental analysis [16]. Label-free

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AuNPs (45 nm) were utilized for the selective detection of Cr6+ ions, and were characterized using zeta potential, XRD and DLS. The zeta potential (ζ) of the AuNPs was −29.8 mV, which indicated a high degree of stability. Furthermore, ζ of Cr6+-AuNPs was about −0.2 mV, which clearly demonstrated that the AuNPs aggregated in the presence of Cr6+ due to coordination with two citrate molecules on their surface of the AuNPs (Fig. S1) [15].

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The XRD pattern (Fig. S2) exhibited characteristic peaks at 38.28˚ (111), 44.46˚ (200), 64.68˚ (220), 77.63˚ (311) and 81.79˚ (222), which corresponded to a face centered cubic (FCC) structure (JCPDS card number: 04-

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0784). The average size of the AuNPs was ~47 nm, as calculated using the Scherrer equation [28]; this was similar to the particles sizes measured by DLS and TEM (Fig. 1 and 2). Upon addition of Cr6+, the UV-Vis

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spectrum of AuNPs showed a decrease in the absorption band at 530 nm and an increase in a new band at 750 nm (Fig. 1). The absorption at 750 nm could have originated from the coupled SPR band, owing to the

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proximity to adjacent nanoparticles resulting from the aggregation of AuNPs. The AuNP solution rapidly changed color from wine red to violet upon addition of Cr6+, indicating that the Cr6+ ions bound to citrate,

(Fig. 1).

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leading to a shorter distance between adjacent AuNPs [29,30]; Cr6+-AuNPs, as noted by the band at 697 nm

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30, 45, 55, and 70 nm AuNPs were prepared using TSC concentrations of 2.5, 2.2, 1.8, and 1.4 mM,

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respectively. Photographic and TEM images of the AuNPs upon addition of Cr6+ (or Cr3+) ions are displayed in Fig. 2. The 30-nm AuNPs did not aggregate upon addition of Cr6+ or Cr3+, whereas the 55- and 70-nm AuNPs aggregated upon addition of both Cr3+ and Cr6+, showing no selectivity. Notably, the 45-nm AuNPs aggregated upon addition of Cr6+, but not Cr3+. The selective aggregation in the presence of Cr6+ was confirmed by TEM, and the color change of the AuNP solution. The 45-nm AuNPs showed selectivity towards Cr6+ over Cr3+, which originated from the surface curvature of the AuNPs, and was associated with the orientation of citrate ions capped on the surface of the AuNPs. The citrate ions capped on the surface of the 45-nm AuNPs must be oriented favorably to coordinate well with Cr6+ ions. Kim et al. reported that the formation of nanoparticle aggregates was based on the size-dependent interparticle interaction energy [31]. The energy-dependent selectivity towards Cr6+ over Cr3+ will be discussed in section 3.5.

3.2. Spectrophotometric detection of Cr6+ using 45-nm label-free AuNPs

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The wine red, colloidal label-free AuNPs solution was well dispersed, with a mean size of 45 nm, and strong LSPR at 530 nm. The aggregations induced by the complex formation between Cr6+ and the 45-nm AuNPs was monitored as a function of Cr6+ using photographic images and UV-Vis absorption (Fig. 3). The localized SPR absorbance of the AuNP solution at 530 nm decreased upon addition of Cr6+ ions and a new band appeared at 750 nm, resulting from the AuNP aggregation. The A750/A530 absorbance ratio increased linearly with increasing

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concentration of Cr6+, with a correlation coefficient of 0.9807 and LOD of 0.4 nM. The LOD of Cr6+ ions was

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found to be 4 nM using the naked eye.

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3.3. Cr6+ binding site on aggregated 45-nm AuNPs

Aggregation of the Cr6+-AuNPs was visualized using photographs, DLS, and TEM (Figs. 1 and 2). The exact

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binding nature was evaluated using XPS. Fig. 4 shows wide scan spectra of AuNPs and Cr6+-AuNPs, and highresolution peaks corresponding to their O1s binding energies. The high resolution peak in the O1s binding

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energy region of label-free AuNPs was deconvoluted into four peaks, which appeared at 529.7, 531.7, 532.3, and 533.5 eV, and corresponded to Au-O, -OH, C-O−, and −O-C=O, respectively [31]. The O1s peak in Cr6+-

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AuNPs was deconvoluted into seven peaks, at 529.7, 530.3, 531.7, 532.0, 532.3, 532.8 and 533.5 eV, and

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corresponded to the bonded oxygen atoms of Au-O, O=Cr, -OH, Cr-OH, C-O−, Cr-O, and −O-C=O, respectively [32−34]. The presence of Cr-O bonds at 530.3, 532, and 532.8 eV clearly showed that Cr6+ ions were bound to oxygen atoms of the citrate moiety, leading to AuNP aggregation. Generally, Cr6+ ions form octahedral complexes with six coordinate ligands. In this probe, Cr6+ ions bound to two citrate molecules, with each citrate molecules providing one hydroxyl and two carbonyl groups, which lead to AuNP aggregation (Scheme 1) [15]. The Cr-O binding nature in Cr6+-AuNPs was further characterized with TOF-SIMS. The mass spectra of label-free AuNPs and Cr6+-AuNPs are shown in Fig. 5. The mass peaks at m/z 67.93, 68.94, 167.86, 95.94, 100.95, and 167.94 corresponded to CrO+, CrOH+, Cr2O4−, CO2Cr−, C2H2O2Cr−, and C4H4O4Cr− fragments, respectively. These mass peaks did not appear in the mass spectrum of the label-free AuNPs, clearly showing that the Cr6+ ions were bound to oxygen atoms on the surface of AuNPs.

3.4. Cr6+ selectivity and interference

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The selectivity of the sensor upon addition of 1 mM of various metal ions (Cr3+, Co3+, Al3+, Mg2+, Fe3+, Li+, Ba2+, As3+, Cu2+, Ni2+, Ca2+, Na+, Ga3+, Cd2+, Ge4+, Mn2+, and Sn2+) was evaluated using the absorbance ratio of A750/A530 (Fig. 6(A)). Further selectivity studies were performed using various anions, including NO2−, ClO4−, NO3−, SO42−, PO45−, F−, Cl−, Br−, I−, CH3CO2−, C3H5O(COO)33−, and C6H5(COO)−. UV-Vis spectra of the labelfree AuNPs upon addition of anions (1 mM) and the corresponding absorbance ratios (A750/A530) were obtained

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(Fig. 6(B)). Various metal ions and anions did not induce any changes in the colors of the AuNP solutions or UV-Vis spectra. The notable decrease in the absorption band at 530 nm was characteristic for Cr6+ ions and

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could be easily distinguished from that of many other ions. The change in the absorbance ratio (A750/A530) induced by Cr6+ ions was 7−25-fold greater than those of other ions, which suggested a characteristic interaction

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between 45-nm AuNPs and Cr6+. To test for the interference with selectivity towards Cr6+, spectral changes in the sensors with Cr6+ (10 nM) were evaluated in the presence of 1 mM of other ions. The additional ions did not

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interfere with the determination of Cr6+, even when the concentrations were much higher than that of Cr6+ (Fig. S3). Therefore, the proposed method could be utilized for the selective determination of Cr6+, even in the

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presence of high-concentrations of possible interfering substances.

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3.5. DFT calculations of selectivity towards Cr6+ over Cr+3 by label-free AuNPs

Ab initio calculations were carried out for Cr6+-AuNPs and Cr3+-AuNPs to investigate the selectivity towards Cr6+ at the atomic level. As it was impossible to calculate the structure including all of the Cr6+ ions and citrate molecule capped on the surface of AuNPs, a model system was introduced. The calculation was approximated with a Cr6+ ion and two citrate ligands; the most stable conformation and energies were calculated for a Cr6+ (or Cr3+) ion ligated with two citrates (Fig. 7). The binding energy for the Cr6+-citrate complex was −3067 kcal/mol, while that of the Cr3+-citrate complex was −419.3 kcal/mol. The Cr6+ complex was seven times more stable than the Cr3+ complex, which explained the increased affinity towards Cr6+ over Cr3+. These results correlated with the experimentally observed selectivity towards Cr6+. However, the DFT calculations did not explain why the selectivity was size-specific to the 45-nm AuNPs.

3.6. Optimum conditions for Cr6+ detection

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As the detection of Cr6+ ions depended on the size of the AuNPs, the stability and sensitivity of the AuNPs were examined at various pH values, salt concentrations, and reaction times. The assay was severely affected by the solution pH, as pH affected interaction between AuNPs and Cr6+ ions, and stability of the AuNPs. The absorbance ratios (A750/A530) of AuNPs and Cr6+-AuNPs were examined at pH 4−9 (Fig. 8(A)). At pH 6 and 7, the AuNPs seemed the most stable in terms of the absorption ratio (A750/A530), while the sensitivity was the

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highest at pH 6. Thus, pH 6 was determined to be optimal, due to the improved stability and the sensitivity. The absorbance ratios (A750/A530) of AuNPs and Cr6+-AuNPs as a function of NaCl concentration are shown

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in Fig. 8(B). The absorbance ratios (A750/A530) of Cr6+-AuNPs was the highest, while that of AuNPs remained constant up to 0.05 M NaCl. Therefore, the optimal NaCl concentration was 0.05 M.

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We examined the aggregation kinetics of label-free AuNPs with various concentrations of Cr6+ by measuring the UV-Vis absorption ratio (A750/A530); the reaction was found to require ~70 min for completion, regardless of

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the concentration of Cr6+ (Fig. 8(C)).

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3.7. Application of AuNP sensor for determination of Cr6+ in real water samples

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Cr6+ was determined in tap, pond, and waste water samples spiked with 5 and 9 nM of Cr6+ to examine the

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practical applications of the developed assay. The Cr6+ contents in real samples were measured using the 45-nm label-free AuNP probe and ICP-OES. As shown in Table 1, the analytical results of the proposed assay were the same as those measured by ICP-OES. The probe could be utilized to monitor Cr6+ ions in aqueous samples, and the developed method was superior to instrumental methods used currently, in terms of simplicity, cost, and time. Furthermore, this sensor showed better sensitivity than earlier nanoparticle sensors (Table 2)

4. Conclusions

The sensor was developed to monitor Cr6+ ions, and the pH, salt concentration, and response time were optimized. The selectivity of the sensor towards Cr6+ was only observed with 45 nm AuNPs, which was explained by the binding energies between the Cr6+ (or Cr3+) ions and citrate ligands using DFT calculations. The binding site was characterized by XPS and TOF-SIMS; Cr6+ ions bound to six oxygen atoms from two citrate ligands, which induced AuNP aggregation.

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The LOD of the sensor was found to be 0.4 nM, making it the most sensitive assay for Cr6+ ions developed to date. Furthermore, this probe was selective for Cr6+ ions in the presence of high concentrations of other ions. Therefore, in terms of simplicity, sensitivity and selectivity, this method is superior to previously reported nanoparticle sensors.

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Acknowledgements

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This research was financially supported by the Korea Institute of Science and Technology (grant number, 2E27070] and the Korea Ministry of Environment (grant number, 2016000160008) as a "Public Technology

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Program based on Environmental Policy".

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[29] W. Hergert, T. Wriedt, The Mie theory, Springer series in optical sciences, Berlin Heidelberg, 2012.

[30] S. Jeon, T. Thajudeen, C.J. Hogan, Evaluation of nanoparticle aggregate morphology during wet milling, Power Tech. 272 (2015) 75−84.

[31] T. Kim, K. Lee, M.S. Gong, S.W. Joo, Control of gold nanoparticles aggregates by manipulation of interparticle interaction, Langmuir 21 (2005) 9524−9528. [32] H. Tsai, E. Hu, K. Perng, M. Chen, J.C. Wu, Y.S. Chang, Instability of gold oxide Au2O3, Surf. Sci. 537 (2003) L447−L450. [33] B.A. Manning, J.R. Kiser, H. Kwon, S.R. Kanel, Spectroscopic investigation of Cr(III) and Cr(VI) treated nanoscale zerovalent iron, Environ. Sci. Technol. 41 (2007) 586−592.

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[34] M. Jelinek, J. Zemek, M. Vandrovcova, L. Bacakova, T. Kocourek, J. Remsa, P. Pisarik, Bonding and bio properties of hybrid laser/magnetron Cr-enriched DLC layers, Mat. Sci. Eng. C, 58 (2016) 1217−1224.

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Biographies

Rajalakshmi Kanagarajreceived her B.S. in 2009 at chemistry of Cardamon Planter’s Association

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College and her PhD in 2016 under Prof. Abraham John investigating fabrication, and

characterization of functionalized multiwalled carbon nanotubes-polymer composite films. She is

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currently a postdoctoral researchfellow at the Korea Institute of Science and Technology (KIST) with

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research interestsin design, and fabrication of nanoparticle sensor.

Yun-Sik Nam is a senior research scientist working in Korea Institute of Science and Technology,

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nanoparticle sensor.

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Seoul, Republic of Korea. His research interests cover analytical chemistry, biosensor, and

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Sung Jin Paiis a postdoctoral research fellow at the Korea Institute of Science and Technology (KIST) with research interestsin the field of theoretical calculations on molecular structure.

Sang Soo Hanis a principal investigatorat the Korea Institute of Science and Technology (KIST) with research interestsin the field of theoretical calculations on molecular structure.

Kang-Bong Lee is a principal investigatorand a professor in Korea Institute of Science and Technology, Seoul, Republic of Korea. Hiscurrent research includes fabrication and designforvarious fluorescence sensorsand colorimetric nanoparticle sensors.

Scheme 1.

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Scheme 1. Proposed mechanism for the aggregation of 45 nm label-free AuNPs in the presence of Cr6+.

(B)

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Fig. 1.

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Fig. 1. UV-Vis absorption spectra of label-free AuNPs solutions (A) in the absence of Cr6+ and (B) in the presence of Cr6+ ions. Insets: Photographic images and DLS measurements of AuNPs (A) in the absence of Cr6+

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and (B) in the presence of Cr6+

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Fig. 2

Fig. 2. HR-TEM images obtained for AuNPs, AuNPs in the presence of Cr6+, and AuNPs in the presence of Cr3+, the AuNPs were of the following sizes: (A) 30-nm, (B) 45-nm, (C) 55-nm, (D) 75-nm. Insets: Corresponding photographic images.

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Fig. 3

Fig. 3. (A) Photographic images of the color change of label-free AuNPs upon addition of Cr6+ ions with various concentrations (0, 1, 2, 3, 4, 5, 6, 7 and 8 nM). (B) UV-Vis absorption spectra of label-free AuNPs upon addition of Cr6+ ions with various concentrations (0, 1, 2, 3, 4, 5, 6, 7, and 8 nM). Inset: Plot of A750/A530 vs. Cr6+ ion concentration.

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Fig. 4

Fig. 4. Wide scan XPS spectra obtained for (A) label-free AuNPs, and (B) Cr6+-AuNPs. High resolution and deconvoluted XPS peaks in the binding energy range of the O1s signal in (C) label-free AuNPs and (D) Cr6+AuNPs.

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Fig. 5.

Fig. 5. Mass peaks for (A) CrO+, (B) CrOH+, (C) Cr2O4−, (D) CO2Cr−, (E) C2H2O2Cr−, and (F) C4H4O4Cr− fragments in TOF-SIMS spectra of label-free AuNPs (red line) and Cr6+-AuNPs (blue line). These molecular fragments were expected based on Cr6+-AuNP structural elements.

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Fig. 6.

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Fig. 6. UV-Vis absorption spectra and corresponding absorbance ratios (A750/A530) of label-free AuNPs upon addition of 1 mM Cr6+ and (A) other metal ions ((1) Cr3+, (2) Co3+, (3) Al3+, (4) Mg2+, (5) Fe3+, (6) Li+, (7) Ba2+, (8) As3+, (9) Cu2+, (10) Ni2+, (11) Ca2+, (12) Na+, (13) Ga3+, (14) Cd2+, (15) Ge4+, (16) Mn2+, and (17) Sn2+ ions), or (B) anions ((1) NO2−, (2) ClO4−, (3) NO3−, (4) SO42−, (5) PO43−, (6) F−, (7) Cl−, (8) Br−, (9) I−, (10) CH3CO2−, (11) C3H5O(COO)33−, and (12) C6H5(COO)−).

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Fig. 7.

Fig. 7. Most stable conformations and binding energies for (A) Cr3+-citrate complex and (B) Cr6+-citrate

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complex using DFT calculations.

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Fig. 8.

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Fig. 8. UV-Vis absorbance ratio (A750/A530) for label-free AuNPs and Cr6+-AuNPs at (A) various pH values (4−9) and (B) various NaCl (0−0.1 M), (C) Time-dependent absorbance ratio (A750/A530) over 100 min in the presence of various Cr6+ ion concentrations (0−8 nM).

Table 1 Determination of Cr6+ ions added to tap water, pond water and waste water samples using a probe of 45-nm label-free AuNPs compared with ICP-OES analysis (n = 6).

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Colorimetric probe Added amount (nM)

Detected amount (nM)

Recovery (%)

5

5.01

100.20

9

8.98

99.77

9 ± 0.02

5

4.91

98.20

5 ± 0.09

9

8.97

5

5.02

9

9.014

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Pond water

99.66

9 ± 0.03

100.4

5 ± 0.02

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Wastewater

5 ± 0.01

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Tap water

Detected amount (nM)

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Sample

ICP-OES

9 ± 0.01

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100.15

Table 2 Comparison of colorimetric nanoparticle sensors for the detection of Cr6+ ions reported in the literature.

No

Capping Ligand

Nanoparticle type

LDRa (µM)

LODb (nM)

Ref.

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0.1−0.6

20

[9]

2

CTABc

AuNRsd

0.1−20

88

[10]

3

Label-free

AgNPs

0.001−1000

1

[11]

4

CTAB

Agcore–Aushell

1−8

10

[12]

5

Leaf extract

AgNPs

1−10000

1000

[13]

6

Tween 20

AuNPs

0−3.5

7

1,5-Diphenylcarbazide

AuNPs

0.05−20

8

Tartaric acid

AgNPs

0.2−20

9

Label-free

AuNPs

0.001−0.009

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[16]

0.4

This work

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HIGHLIGHTS

[14]

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AuNRs; Gold nanorods

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LOD; Limit of detection

CTAB; Cetyltrimethy ammoniumbromide

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AuNPs

LDR; Linear dynamic range

b c

1,4-Dithiothreitol

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a

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•Color change is induced by size-specific Au nanoparticles aggregation in the presence of Cr6+ ions. •A newly developed assay method uses 45-nm label-free Au nanoparticles for detection of Cr6+ions. •The sensor exhibits excellent selectivity for Cr6+ions over other metal cations and anions. •The present assay method can detect a Cr6+ionic concentration of ~ 0.4nMwithin 70min.

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