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Designing of glutathione-lactose derivative for the fabrication of gold nanoclusters with red fluorescence: Sensing of Al3+and Cu2+ ions with two different mechanisms
Optical Materials 100 (2020) 109704
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Designing of glutathione-lactose derivative for the fabrication of gold nanoclusters with red fluorescence: Sensing of Al3þand Cu2þ ions with two different mechanisms Mehul R. Kateshiya a, Naved I. Malek a, Z.V.P. Murthy b, Suresh Kumar Kailasa a, * a b
Applied Chemistry Department, S. V. National Institute of Technology, Surat, 395 007, India Chemical Engineering Department, S. V. National Institute of Technology, Surat, 395 007, India
A R T I C L E I N F O
A B S T R A C T
Keywords: GSH-lactose-AuNCs UV–Visible and fluorescence spectroscopy FT-IR HR-TEM Al3þand Cu2þ ions
In this work, a simple and novel glutathione-lactose (GSH-lactose) derivative was synthesized and used as a template for the fabrication of fluorescent gold nanoclusters (GSH-lactose-AuNCs). The synthesized fluorescent GSH-lactose-AuNCs were stable and showed red emission color by irradiating with 365 nm of UV light. At 410 nm of excitation wavelength, the fluorescent GSH-lactose-AuNCs exhibited a strong emission peak at 635 nm. The fluorescent GSH-lactose-AuNCs acted as a fluorescent sensor for the rapid and sensitive detection of two metal ions i.e., Al3þ and Cu2þ ions with different sensing mechanisms such as fluorescence enhancement for Al3þ ion and fluorescence quenching for Cu2þ ion). As a result, the emission peak of GSH-lactose-AuNCs at 635 nm exhibited independent behaviour i.e., a linear enhancement with the addition of Al3þ ion and a linear quenching efficiency with Cu2þ ion. Thus, good linear relationships were noticed between intensity (I and I0/I) over the concentration ranges of 0.075–100 μM for Al3þ ion and of 0.050–100 μM for Cu2þ ion with correlations co efficients of 0.9962 and 0.9964. The detection limits are 12 and 52 nM for Al3þ and Cu2þ ions. The GSH-lactoseAuNCs-based analytical approach was successfully applied to quantify Al3þ and Cu2þ ions in real samples.
1. Introduction Recently, gold nanoclusters (AuNCs) have received a great deal of scientific attention because of their outstanding applications in applied sciences (sensing, bioimaging, catalysis, therapy, and electronics) [1,2]. AuNCs exhibit unique physicochemical properties that are specifically varied from their large size materials (AuNPs), resulting to possess quantum confinement effects, which is due to the presence of precise Au atoms in each cluster (~150 atoms) and ultrasmall size (<2 nm) [3]. Notably, AuNCs exist with discrete electronic structures that allow them to behave as molecular-like properties, including quantized charging, electronic transitions, molecular chirality and magnetism, respectively [4,5]. Importantly, an one atom difference in AuNCs leads to drastic changes in their optical and physico-chemical properties, revealing their atom- and size-dependent properties [6]. The number of atoms in AuNCs has been controlled by introducing novel ligands as templates for the fabrication of atomically precise AuNCs [7]. Thus, ligand chemistry has been evolved as an emerging research area for understanding the structure and properties of AuNCs and the construction of AuNCs with
novel architecture and desired analytical applications [2]. Ligand chemistry plays a vital role in fabricating atomically precise AuNCs for specific interactions with other chemical species [8–12]. In view of this, various ligands including, proteins, amino acids, egg-white, and small organic molecules have been used as templates for the fabri cation of AuNCs [13–16]. Notably, ligands on the surfaces of AuNCs would offer unique features such as protection of AuNCs, the formation of monodisperse, tuning of surface and optical properties [2]. However, some of the issues (introduction of cost-effective novel ligand, mono dispersity, and high quantum yield) are still unresolved in the chemical approaches for the fabrication of AuNCs using various ligands as pro tecting ligands [17]. Aluminium (Al3þ) has found to be one of the most abundant element in the lithosphere. Alloys of aluminium have been widely used to pro duce distinct structural materials and their uses in various industrial and scientific sectors [18]. Although aluminium alloys and its composites have shown notable advance applications in different areas, exceeding levels of Al3þ ion can cause multiple diseases such as bone softening, breast cancer, Alzheimer’s and Parkinson’s [19,20]. Importantly, a
* Corresponding author. E-mail addresses: [email protected], [email protected] (S.K. Kailasa). https://doi.org/10.1016/j.optmat.2020.109704 Received 11 November 2019; Received in revised form 10 January 2020; Accepted 14 January 2020 0925-3467/© 2020 Elsevier B.V. All rights reserved.
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higher amount of Al3þ ion in the environment is significantly damaged the growth of plants. As per the world health organization, Al3þ ion should be in the range of 7 mg kg/week or 3–10 mg/day for humans [21]. For quantification of Al3þ ion in environments, serval analytical techniques like high-performance liquid chromatography, nuclear ab sorption spectroscopy, atomic absorption spectrometry and mass spec trometry have been adopted to establish analytical procedures for the trace level identification of Al3þ ion [22–26]. Similarly, copper ion (Cu2þ ion) has drawn considerable attention because of its beneficial or dangerous environmental and human health impacts [27]. It has high bio-accumulative nature, causing the inter ruption of cellular balance and serious diseases (Wilson’s and Menke’s diseases) [28]. Various analytical methods have been adopted for Cu2þ ion analysis with elevated selectivity [29,30]. Among the analytical techniques, fluorescence spectrometry has been recognized as one of a cost-effective analytical tool for the analysis of metal ions [31]. For example, Liu’s group described a fluorescence method for identification of Al3þ ion using naphthol aldehyde-tris derivate as a probe via fluo rescence “turn-on” mechanism [32]. Gupta et al. synthesized 1-(2-pyr idylazo)-2-naphthol derivated for selective colorimetric and fluorescence recognization of Al3þ ion [33]. Schiff base receptors have been prepared and used as fluorescent sensors for the analysis of Al3þ ion [34]. A fluorescence-based optical probe was developed for the quantitative analysis of Al3þ ion [35]. Similarly, different fluorescent probes such as DNAzymes, protein, luminescent metal clusters and organic dyes have been reported as promising sensors for assaying of Al3þ and Cu2þ ions [36–38]. It was observed that the assay of Al3þ ion was based on various mechanisms including energy transfer [39,40], charge transfer [41] and chelation enhance fluorescence [35,42]. These adopted procedures have shown to be sensitive methods for the trace level analysis of Al3þ ion, however, some of the methods require highly skilled persons to operate the instruments and inability to on-field monitoring. Furthermore, they required either specific organic ligands or modifications for the selective capturing of Al3þ ion from various environments. In recent years, AuNCs have been elevated as remarkable fluorescent probes for the analysis of various metal ions [43]. For example, Hg2þ, Agþ, Cr3þ, Cr6þ, Zn2þ, Al3þ, Cd2þ and Pb2þ ions have
been recognized by using fluorescent nanoprobes including AuNCs [44–53]. In these methods, various ligands such as glutathione, metal-organic framework, dimercaptosuccinic acid, and cytidine have been used as templates for the preparation of AuNCs. In this work, a novel ligand glutathione-lactose derivative (GSHlactose) was synthesized by the reaction between GSH and lactose via glycosylamine formation (Scheme S1 of Supporting Information). The synthesized GSH-lactose derivative was the first time used as a novel ligand for one-pot synthesis of ultra-small red fluorescent AuNCs (Scheme 1). The GSH-lactose-AuNCs exhibited an intense emission at 635 nm by them exciting at 410 nm. The GSH-lactose-AuNCs showed typical independent behaviour i.e., fluorescence enhancement with Al3þ and quenching with Cu2þ ions, enabling to establish a facile analytical method for parallel detection of Al3þ and Cu2þ ions with lower detection limits. To our knowledge, this ligand is the first report on the synthesis of AuNCs for parallel fluorescence detection of Al3þ and Cu2þ with two different mechanisms. Combining the simplicity in the synthesis of GSHlactose derivative and GSH-lactose-AuNCs, the proposed method has proven to be a gold-standard platform for the detection of Al3þion in the antacid drug tablet and of Cu2þion in copper oxychloride (fungicide) and industrial wastewaters. 2. Experimental section 2.1. Chemicals and materials HAuCl4⋅xH2O, glutathione in the reduced form (GSH) and metal salts were procured from Sigma-Aldrich, USA. Lactose was procured from SRL Chemicals LTD, India. Antacid tablets were purchased from a local medical store, Surat, India. Copper oxychloride 50% WP was purchased from Prakash Sales Agencies, Surat, India. 2.2. Instrumentation The absorption spectra of GSH-lactose-AuNCs were recorded on a Maya Pro 2000 optical spectrophotometer, Ocean Optics, USA. The excitation and emission spectra of GSH-lactose-AuNCs were recorded on
Scheme 1. Schematic represent of GSH-lactose-AuNCs formation with the help of GSH-lactose derivative. 2
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a Cary Eclipse fluorescence spectrometer (Agilent Technologies, USA). The morphology and size of GSH-lactose-AuNCs were confirmed on high-resolution transmission electron microscopy (HR-TEM) (JEM2100, JEOL, Japan). The functional groups of GSH-lactose-AuNCs were recorded by using Fourier transform infrared (FT-IR) spectrometry (FTIR APHA ІІ, Bruker, Germany).
3. Results and discussion 3.1. Spectral characterization of GSH-lactose derivative and GSH-lactoseAuNCs First time, we designed a novel ligand for the synthesis of AuNCs using carbohydrate chemistry and small biomolecule. The bottom-up strategy was carried out for the preparation of GSH-lactose-AuNCs where GSH-lactose derivative acted as a template and reducing agent. The present study provides the following objectives (i) synthesis of GSHlactose derivative with one-step reaction, (ii) controlled growth of AuNCs with fluorescence properties and (iii) development of a facile analytical platform for the detection of trace-level metal ions with two different mechanisms. Briefly, the amino group of glutathione was reacting with the aldehyde group of lactose, yielding yellows lacy product (Scheme S1 of Supporting Information). In this, the gold pre cursor is initially formed the complex with GSH-lactose and then controlled reduction of Au3þ ion to Au0 atom. GSH-lactose provides multi-functional active groups, allowing to generate stable AuNCs with good binding sites of AuNCs surface (Scheme 1). The present design describes that the combination of carbohydrate chemistry and small biomolecule to synthesize novel GSH-lactose derivative for the controlled growth of AuNCs with an average size of 1.03 � 0.37 nm. In order to synthesize GSH-lactose derivative with good yield, GSH-lactose derivative was synthesized by using different concentrations of GSH (0.25, 0.50, 1.0 mmol) and lactose (0.75, 1.5, 3.0 mmol) and studied their absorption spectra (Fig. S1 of Supporting Information). The maximum absorbance peak was noticed at 340 nm using GSH and lactose ratio at 1:3 mmol. Thus, GSH-lactose derivative was synthesized by using a 1:3 mol ratio of GSH and lactose. Fig. 1, displays the optical characteristics (UV–visible, excitation, and emission) of lactose-GSH-Au NCs. In the UV–visible spectrum of GSH-lactose-AuNCs, the absorption maximum was observed at 325 nm, revealing the AuNCs did not show any surface plasmon resonance peak in the visible region, which confirms the generation of fluorescent AuNCs. The GSH-lactose-AuNCs exhibited a remarkable emission peak at 635 nm when excited at a wavelength of 410 nm. In daylight, GSHlactose-AuNCs show a clear yellowish-green color whereas it exhibits a strong red emission under UV light of 365 nm wavelength. To establish optimum conditions for AuNCs, the effect reaction time (1–36 h) and the effect of GSH-lactose amounts (10–50 mg/mL) were investigated on the spectral characteristics of AuNCs (Figs. S2 and S3 of Supporting Infor mation). The maximum emission intensity of GSH-lactose-AuNCs was
2.3. Synthesis of GSH-lactose-AuNCs A mixture of GSH (307.3 mg; 1 mmol) and lactose (1081.01 mg; 3 mmol) with a mole ratio of 1:3 was dissolved in deionized water (2 mL). The mixture was sonicated for 10 min and the mixture was subjected to an oil bath at 130 � C for 30 min. Yellowish lacy product was obtained, suggesting the formation of GSH-lactose derivative. The synthesized GSH-lactose derivative was centrifuge at 12,000 rpm for 15 min, after removing the upper layer of solution, this process 3–4 time repeated. The GSH-lactose derivative was stored for further use in the synthesis of GSH-lactose AuNCs. The GSH-lactose-AuNCs were produced by using a derivative of GSHlactose as an encapsulating agent. In short, 50 mg of GSH-lactose de rivative was dissolved into 1.0 mL of deionized water. The as-prepared GSH-lactose derivative solution (50 mg in 1 ml) was added into a re action flask that contains 5.0 mL of 0.25 M HAuCl4 solution and then the mixture was stirred for 12 h. An equal volume of EtOH (6.0 mL) was mixed with the reaction mixture and then centrifuged at 12,000 rpm for 15 min. The obtained GSH-lactose-AuNCs precipitate was redispersed in Milli-Q water for further characterization and applications. 2.4. Detection of Al3þ and Cu2þ ions To develop a fluorescence analytic approach for assaying of Al3þ and Cu ions, 1.0 mL (0.75 mM) of GSH-lactose-AuNCs was taken into a small glass vial and then added 0.5 mL of various amounts of Al3þ ion (0.075–100 μM) and Cu2þion (0.050–100 μM). The specimens were vortexed for 60 s and kept standing for a few minutes. The emission intensities of solutions were noted by exciting at 410 nm. Further, different common ions (Cd2þ, Mn2þ, Co2þ, Cr3þ, Hg2þ, Ni2þ, Pb2þ, Ca2þ, and Zn2þ,1 mM) were investigated to evaluate the selectivity of the probe. 2þ
2.5. Analysis of Al3þ and Cu2þ ions in real sample GSH-lactose-AuNCs were applied as a probe for the analysis of both Al3þ and Cu2þ ions in real samples (drug, fungicide and industrial wastewater). Antacid drug tablets were purchased from Local Medical Store, Surat, Gujarat, India. The tablets were powdered and 5 mg of tablet powder was taken into a 50 mL standard flask, sonicated for 5 min and then diluted with water up to the mark. The solution was filtered and the amount of Al3þ ion was estimated by using the above procedure. Copper oxychloride is used as a fungicide in many vegetables, and it contains 50% (w/w) of copper oxychloride. To quantify Cu2þ ion in fungicide, 5 mg of copper oxychloride was dissolved in 50 mL of deionized water with a slight addition of dilute NaOH. From this, different concentrations of copper oxychloride (50, 100, 200 nM) were prepared and estimated Cu2þ ion concentration as per the described procedure earlier. To evaluate the practical application of the method, the amount of Cu2þ ion was estimated in industrial wastewater, Chamuda Brass Products Pvt, Ltd, Jamnagar, Gujarat, India. The collected wastewater was filtered with Whatman filter paper to remove the microsize parti cles. The Cu2þ ion was quantified in industrial wastewater as per the described procedure in the above section.
Fig. 1. UV–visible spectra of GSH-lactose, fluorescence excitation (at 410 nm) and emission (at 635 nm) spectra of GSH-lactose-AuNCs (Inset: the image of GSH-lactose-AuNCs at under UV lam at 365 nm and daylight). 3
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GSH at 8.6 ppm was disappeared in the 1H NMR spectrum of GSHlactose derivative, confirming the formation of GSH-lactose derivative. Fig. S5a of Supporting Information shows that the FT-IR spectral characteristics of pure GSH. The characteristic GSH absorption bands around 1713-1613 cm 1 belong to –COO– group (symmetric stretching). The peaks at 1396 and 1713 cm 1 are generated due to –COO– (asym – O (stretching). The peak of –OH of COOH metric stretching) and C– group is noticed at 1279 cm 1 and the two peaks at 3350-3027 cm 1 are corresponded to -N-H stretching (symmetric and asymmetric). The characteristic –SH group stretching of GSH was appeared at 2524 cm 1. Fig. S5b of Supporting Information displaces that the FT-IR spectrum of pure lactose. A broad peak was observed at 3528-3100 cm 1 which is due to poly hydroxyl (-OH) groups of lactose. The peaks at 2900 and 1657 cm 1 belong to–CH- group stretching and -C-O- six members cyclic ring. The GSH-lactose derivative FT-IR spectrum was shown in
noticed at 635 nm using reaction time at 12 h and 50 mg/mL of GSHlactose. Thus, GSH-lactose-AuNCs are synthesized using 50 mg/mL of GSH-lactose at 12 h as reaction time. To determine functional groups of GSH-lactose derivative, 1H NMR and FT-IR spectra were studied. Fig. S4a of Supporting Information shows that the 1H NMR of pure GSH, the peak at 8.6 ppm corresponds to the protons of –NH2 and the peak at 8.4 ppm belongs to the protons –NH–. The peaks at 4.4–3.7 ppm are attributed to –CH– protons of GSH. The peaks at 2.8–2.2 ppm correspond to –CH2 protons. Fig. S4b of Supporting Information shows that the 1H NMR of pure lactose. The peaks at 6.3–5.1 and 2.6 ppm belong to the –CH– protons of lactose. The peaks at 4.8–3.6 ppm correspond to protons of –OH of lactose. Fig. S4c of Supporting Information shows that the 1H NMR of GSH-lactose deriva tive. It can be clearly noticed that the peaks of –NH- group of GSH was appeared at 8.0–5.6 ppm, however the characteristic NH2 group peak of
Fig. 2. HR-TEM images of GSH-lactose-AuNCs at (a) 2 nm scale bar (b) lattice fringes of GSH-lactose-AuNC at 2 nm scale bars, (c) histogram of GSH-lactose-AuNCs drawn at 1 nm scale bar. HR-TEM images of GSH-lactose-AuNCs with (d) Al3þ ion and (e) Cu2þ ion. Histograms drawn at 1 nm scale for GSH-lactose-AuNCs with (f) Al3þ ion and (g) Cu2þ ion. 4
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Supporting Information Fig. S5c. The broad peak at 3351-3293 cm 1 belongs to–NH and –OH (stretching) groups. The C–H stretching was observed at 2931 cm 1. The peaks at 2524, 1673 and 1078 cm 1 are generated due to –SH and -C-O- stretchings of six member ring and–NH bending. Fig. S5a of Supporting Information shows that the FT-IR spectrum of GSH-lactose-AuNCs. The characteristic functional groups (-COOH, –NH2, and –OH) of GSH-lactose are either slightly shifted or disappeared due to the formation of GSH-lactose-AuNCs. Noticeably, the stretching peak of –SH group of GSH was completely disappeared due to Au3þ ion-GSH-lactose complex formation. To complement the above spectroscopic characterizations and to estimate the actual size of GSH-lactose-AuNCs, HR-TEM was studied to examine the size and morphology of GSH-lactose-AuNCs (Fig. 2a–c). Fig. 2b shows that the lattice fringes (d ¼ 0.12 nm) spacing is matched with the face-centred cubic (fcc) with (111), demonstrating the crys talline nature of GSH-lactose-AuNCs. The as-synthesized GSH-lactoseAuNCs exhibited ultrasmall size dispersity with an average size of 1.03 � 0.37 nm (Fig. 2c), favouring molecular-like absorption characteristics, which allow them to act as ideal probes for molecular recognition. The quantum yield (QY) of GSH-lactose-AuNCs was estimated with the help of Rhodamine B as a reference and the QY of GSH-lactose-AuNCs is found be 74.48%. The as-synthesized GSH-lactose-AuNCs was very good stability up to 195 days (Fig. 6S of Supporting Information).
investigated on the fluorescence emission intensity of GSH-lactoseAuNCs in the presence of both metal ions (Al3þ ion - 500 μM and Cu2þ ion - 500 μM) (Fig. S7 of Supporting Information). It was noticed that the fluorescence emission intensity of GSH-lactose-AuNCs was gradually increased with increasing time from 0.5 to 7.0 min and then reached to plateau at 4.0 min. Similarly, at 1.0 min, the fluorescence emission intensity of GSH-lactose was maximum quenched with the addition of Cu2þ ion. Thus, 4.0 and 0.5 min were selected as optimum time for the measuring the fluorescence emission spectra of GSH-lactoseAuNCs with Al3þ and Cu3þ ions. To explain possible sensing mechanism of the method, we carried out HR-TEM studies of GSH-lactose-AuNCs in presence of Al3þ and Cu2þ ions (Fig. 2d–g). In HR-TEM of GSH-lactose-AuNCs, the average sizes of GSH-lactose-AuNCs were increased from 1.03 � 0.37 to 6.45 � 0.28 and 5.05 � 0.18 nm with the addition of Al3þ and Cu2þ ions, demonstrating the aggregation of GSH-lactose-AuNCs. Furthermore, the fluorescence quenching of GSH-lactose-AuNCs by Cu2þion was also evaluated using the Stern-Volmer equation [15]. I0/I ¼ 1 þ Ksv [Q] Where I0 and I fluorescence intensities of AuNCs with and without Cu2þ ion. Ksv is the Stern-Volmer quenching constant and [Q] is the concen tration of Cu2þ ion. The Stern-Volmer equation shows good linearity with increasing concentration of Cu2þ ion (Fig. S8b of Supporting In formation), revealing the dynamic or static quenching in the fluores cence emission of AuNCs with Cu2þ ion. To confirm further, timecorrelated single-photon counting was studied by using the fluores cence lifetime of GSH-lactose-AuNCs with and without Cu2þ and Al3þions (Fig. S9 of Supporting Information). The GSH-lactose-AuNCs exhibited an average lifetime as 11.70 ns, however it is decreased to 10.60 ns with the addition of Cu2þ ion. These life-time spectra revealed that the formation of a complex between GSH-lactose-AuNCs and Cu2þion, which results to quench the emission of AuNCs. Similarly, the average lifetime of GSH-lactose-AuNCs was increased to 14.68 ns with the addition of Al3þ ion, illustrating the sensing mechanism for Al3þ ion is based on the aggregation-induced emission.
3.2. Analysis of Al3þ and Cu2þ ions and sensing mechanism The fluorescence emission behaviour of GSH-lactose-AuNCs was evaluated by the addition of various trace metal ions. Fig. 3 shows that the emission spectrum of GSH-lactose-AuNCs at the excitation wave length of 410 nm. After being independent treatment with various metal species (Cu2þ, Al3þ, Cd2þ, Mn2þ, Co2þ, Cr3þ, Hg2þ, Ni2þ, Pb2þ, Ca2þ, and Zn2þ, 1 mM), the maximum emission peak at 635 nm underwent a remarkable enhancement for Al3þ ion and a drastic quenching for Cu2þ ion, whereas other metal ions did not induce any obvious fluorescence enhancement or quenching. Thus, GSH-lactose-AuNCs could be acted as a promising probe for assaying of Al3þ and Cu2þ ions by using fluores cence spectrometry. To expand the sensing ability of GSH-lactose-AuNCs probe for detection of Al3þ and Cu2þions, the effect of reaction time was
3.3. pH study for assaying of Al3þ and Cu2þ ions The pH of the solution has a great impact on the fluorescence behaviour of GSH-lactose AuNCs. The pH of the solution is a very important factor for the detection of target analytes. In view of this, the emission peak intensity of GSH-lactose-AuNCs was investigated in the absence and presence of both ions (Al3þ and Cu2þ) as a function of so dium acetate and phosphate buffer saline (PBS) pH (2.0–12.0). Initially, the fluorescence intensity of GSH-lactose-Au NCs was studied without addition of metal ions (Al3þ and Cu2þ ion) at sodium acetate and PBS pH range from 2.0 to 12.0 (Figs. S10–S11 of Supporting Information). The fluorescence emission intensity of GSH-lactose-Au NCs was drastically changed at both sodium acetate and PBS pHs 2.0 and 12.0. It can be clearly noticed that at sodium acetate and PBS pHs 2.0 and 12.0, the fluorescence intensity was either quenched or enhanced without addi tion of both metal ions. These results demonstrated that these pHs may not be considered as optimum pHs for the detection of both ions. Since, at high acidic pH, active functional groups were protonated, resulting strong charge repulsions and hydrogen bonding, which yields to enhance emission intensity without addition of Al3þ ion. Figs. S10–S11 of Supporting Information exhibit the fluorescence response by the addition of Al3þ and Cu2þ ions at sodium acetate and PBS pHs 2.0–12.0. For Al3þ ion, the fluorescence intensity of GSH-lactose-AuNCs was enhanced maximum at sodium acetate buffer pH 9, showing higher in tensity as compared to the emission spectra at remaining pH (Fig. S10b of Supporting Information). Therefore, an Al3þ ion assay was carried out at sodium acetate buffer pH 9.0 using GSH-lactose-AuNCs as a probe. Similarly, the fluorescence spectrum of GSH-lactose-AuNCs was
Fig. 3. Fluorescence spectra of GSH-lactose-AuNCs after addition of different metal ions (Cu2þ, Al3þ, Cd2þ, Mn2þ, Co2þ, Cr3þ, Hg2þ, Ni2þ, Pb2þ, Ca2þ, and Zn2þ, 1 mM), and (inset: photograph under UV lamp at 365 nm after addition of metal ions). 5
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maximum quenched with Cu2þ ion at PBS pH 9.0 (Fig. S11 of Supporting Information), suggesting the PBS pH 9.0 is optimum pH for assay of Cu2þ ion. The sodium acetate pH 9.0 and PBS pH 9.0 were chosen as optimum pH for the detection of Al3þ and Cu2þ ions using GSH-lactose-AuNCs as a probe.
ion. The limits of detection are 12 and 52 nM for Al3þ and Cu2þ ions, exhibiting much better sensitivity than the reported methods in the literature [54–58] (Table 1). 3.5. Interference study The selectivity of GSH-lactose-AuNCs towards Al3þ and Cu3þ ions was carried out using different chemical species (Fig. 5a and b). Under optimal conditions, the mixture of metal ions (Mg2þ, Cd2þ, Hg2þ, Fe2þ, Ni2þ, Ba2þ, Ca2þ, Cr3þ, Zn2þ, Fe3þ and Pb2þ, 1 mM), anions (Cr2O27 , PO34 , SO24, S2 , F , Cl , Br , and I , 1 mM) and pesticides (chlorprop ham, difenoconazole, fipronil, tebuconazole, metalaxyl, isoproturon and chlorpyrifos, 1 mM) had shown no significant effect on the fluorescence behaviour of GSH-lactose-AuNCs (Fig. 5a and b), however, the fluores cence intensity of GSH-lactose-AuNCs was enhanced with Al3þ ion and quenched with Cu2þ ion even at strong interferents chemical species. These spectral data clearly demonstrated that GSH-lactose-AuNCs acted as an optical sensor for both ions (Al3þ and Cu2þ) in the presence of other competitive chemical species, demonstrating its selectivity for assaying of Al3þ and Cu2þ ions.
3.4. Sensitivity study for Al3þ and Cu2þions To explore the sensitivity of the probe, Al3þ and Cu2þ ions with different concentrations were added independently into the GSHlactose-AuNCs at sodium acetate pH 9.0 and PBS pH 9.0, and their resulting spectra were recorded (Fig. 4a and b) for the calibration graphs construction. For Al3þ and Cu2þ ions: the calibration graphs were con structed by using various concentrations of Al3þ ion (0.075–100 μM) and of Cu2þ ion (0.050–100 μM) (Fig. S8 of Supporting Information). These results indicate that the fluorescence emission of GSH-lactoseAuNCs at 635 nm increases as Al3þ ion concentration raising, and the degree of quenching increases as Cu2þ ion concentration increases. The linear functions of the calibration curves are I ¼ 11.704xþ36.879 (Al3þ ion) with a correlation coefficient (R2) of 0.9962 for Al3þ ion and I0/I ¼ 1.1115xþ0.9847 with a correlation coefficient (R2) of 0.9964 for Cu2þ
3.6. Practical applications 3.6.1. Detection of Al3þion in antacid tablets and industrial wastewater The detection of Al3þ ion in the antacid drug was carried out by using GSH-lactose-Au NCs as a probe. Briefly, each tablet (600 mg) consists 250 mg of Al(OH)3 and Mg (OH)2, 50 mg (active dimethicone) and 50 mg (magnesium aluminium silicate hydrate). Firstly, the standard so lutions of the antacid tablet were prepared by dissolving 50 mg of tablet and then added 1–2 drops of dilute HCl. Then, 3 different concentrations (0.05, 0.06 and 0.07 μg/mL equal to 170, 270 and 330 nM) of the above solution were added into 1 mL of GSH-lactose-AuNCs solution (Fig. S12 of Supporting Information). The amount of Al3þ ion was quantified by the described procedure earlier and results were shown in Table S1 of Supporting Information. Similarly, GSH-lactose-AuNCs was used as a simple optical probe for the analysis of Al3þion in industrial wastewater, which was collected from the manufacturing of Aluminium boxes and bottles in Jamnagar, India. The collected industrial wastewater was filtrated by using Whatman filter paper and then 3 different dilution sample taken and directly added into GSH-lactose-AuNCs solution (Fig. S13 of Supporting Information). The Al3þ ion concentration is found to be 80.23, 100.54 and 210.68 nM and shown in Table S1 of Supporting Information. 3.6.2. Detection of Cu2þion in fungicide (copper oxychloride) and industrial wastewater Copper oxychloride is widely used as a fungicide to control fungi cides in many vegetables and it contains 50% (w/w) of copper oxy chloride. To explore the application of the method, copper oxychloride (50, 100 and 200 nM) solutions were spiked on vegetables (tomato, Table 1 Analytical merits of the present method for fluorescence assays of Al3þ and Cu2þ ion as compared with reported methods in the literature.
Fig. 4. Fluorescence emission spectra of (a) GSH-lactose-AuNCs with different concentration of Al3þ (0.075–100 μM) (inset: photograph under UV lamp at 365 nm after addition of Al3þ (0.075–100 μM)) and (b) GSH-lactose-AuNCs with different concentration of Cu2þ (0.050–100 μM) (inset: photograph under UV lamp at 365 nm after addition of Cu2þ (0.050–100 μM)). 6
Probe
Metal ion
Linear range (μM)
Detection limit (μM)
Reference
Citrate-Au NPs Rhodamine based fluorescent chemosensors Chemosensor Dithiothreitol capped Au NCs Glutathione protected Au NCs GSH-lactose-Au NCs
Al3þ Al3þ
0.5–20
7.4 0.12
[54] [55]
Al3þ Cu2þ
0–60
1.5 80
[56] [57]
Cu2þ
0.1–6.25
86
[58]
Al3þ and Cu2þ
0.075–1 and 0.050–1
0.011 and 0.052
Present work
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allow a facile, fast and reliable analytical method for detection and screening of Al3þ and Cu2þ ions from various samples (food packing materials, tablets, and industrial wastewater). 4. Conclusions In summary, a novel GSH-lactose derivative was designed as a novel organic ligand for the fabrication of fluorescent AuNCs. The GSHlactose-AuNCs acted as a probe for the detection of trace-level detec tion of Al3þ ion via fluorescence enhancement and of Cu2þ ion via fluorescence turn-off mechanisms. The developed probe displayed wider linear ranges of 0.075–100 μM for Al3þ ion and of 0.050–100 μM for Cu2þ ion. The method exhibited lower detection limits, and the probe was successfully applied to assay Al3þ and Cu2þ ion in various samples including food packing material, tablet, fungicide, and industrial wastewater, respectively. Thus, this method could be used as a prom ising analytical method for the quantitative analysis of Al3þ and Cu2þ ions in various samples. Author contributions section Mehul R.Kateshiya, Writing – original draft, He has carried out entire experimental work and written the experimental sections of manuscript, Naved I. Malek, Data curation, He is involved in the discussion of FT-IR spectral data, Z.V.P.Murthy, Data curation, He corrected grammatical mistakes and discussed about TEM data, Suresh Kumar Kailasa, Super vision, Completely supervised the work. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by the Department of Science and Technology, Government of India (EMR/2016/002621/IPC). MK acknowledges the Director, SVNIT, Surat, India for the doctoral fellowship to carry this work.
Fig. 5. Fluorescence emission spectra of GSH-lactose -Au NCs with (a) Al3þ ion upon the addition of various chemical species; inorganic species (metal ions Mg2þ, Cd2þ, Hg2þ, Fe2þ, Ni2þ, Ba2þ, Ca2þ, Cr3þ, Zn2þ, Pb2þ and Fe3þ, 1.0 mM; anions - Cr2O27 , PO34 , SO24, S2 , F , Cl , Br , and I , 1.0 mM) and pesticides (chlorpropham, difenoconazole, fipronil, tebuconazole, metalaxyl, isoproturon and chlorpyrifos, 1.0 mM) inset: photograph under UV lamp at 365 nm and (b) Cu2þion upon the addition of various chemical species; inorganic species (metal ions - Mg2þ, Cd2þ, Ni2þ, Hg2þ, Fe2þ, Ba2þ, Ca2þ, Pb2þ, Cr3þ, Zn2þ and Fe3þ ion, 1.0 mM; anions - Cr2O27 , PO34 , SO24 , S2 , F , Cl , Br , and I , 1.0 mM) and pesticides (chlorpropham, difenoconazole, fipronil, tebuconazole, metalaxyl, isoproturon and chlorpyrifos, 1.0 mM) (Inset: photograph under UV lamp at 365 nm).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.optmat.2020.109704. References [1] R. Jin, C. Zeng, M. Zhou, Y. Chen, Atomically precise colloidal metal nanoclusters and nanoparticles: fundamentals and opportunities, Chem. Rev. 116 (2016) 10346–10413. [2] R.R. Nasaruddin, T. Chen, N. Yan, J. Xie, Roles of thiolate ligands in the synthesis, properties and catalytic application of gold nanoclusters, Coord. Chem. Rev. 368 (2018) 60–79. [3] N. Goswami, Q. Yao, T. Chen, J. Xie, Mechanistic exploration and controlled synthesis of precise thiolate-gold nanoclusters, Coord. Chem. Rev. 329 (2016) 1–15. [4] S. Knoppe, O.A. Wong, S. Malola, H. Hakkinen, T. Burgi, T. Verbiest, C.J. Ackerson, Chiral phase transfer and enantioenrichment of thiolate-protected Au102 clusters, J. Am. Chem. Soc. 136 (2014) 4129–4132. [5] M.A. Aljuhani, M.S. Bootharaju, L. Sinatra, J.-M. Basset, O.F. Mohammed, O. M. Bakr, Synthesis and optical properties of a dithiolate/phosphine-protected Au28 nanocluster, J. Phys. Chem. C 121 (2017) 10681–10685. [6] J. Fang, J. Li, B. Zhang, X. Yuan, H. Asakura, T. Tanaka, K. Teramura, J. Xie, N. Yan, The support effect on the size and catalytic activity of thiolated Au25 nanoclusters as precatalysts, Nanoscale 7 (2015) 6325–6333. [7] C. Yao, J. Chen, M.-B. Li, L. Liu, J. Yang, Z. Wu, Adding two active silver atoms on Au25 nanoparticle, Nano Lett. 15 (2015) 1281–1287. [8] X. Dou, X. Yuan, Q. Yao, Z. Luo, K. Zheng, J. Xie, Facile synthesis of water-soluble Au25-x Agx nanoclusters protected by mono-and bi-thiolate ligands, Chem. Commun. 50 (2014) 7459–7462.
cabbage, and brinjal) and the amount of Cu2þ ion was estimated as per the illustrated procedure earlier. It was noticed that the present method exhibited a good relative standard deviation from 0.21 to 0.63% (Table S2 of Supporting Information). Further, the potential application of the present method was described for the detection of Cu2þ ion in industrial wastewater, Cha muda Brass Products Pvt, Ltd, Jamnagar, India. The collected waste water sample was filtered by Whatman filter paper to remove dust and then the 3 different dilution sample taken and directly added to the GSHlactose-AuNCs solution, the concentration of Cu2þ ion in industrial wastewater was estimated by the described procedure earlier and the data were shown in Table S2 of Supporting Information). The obtained fluorescence spectra were shown in Fig. S14 of Supporting Information. This method was successfully detected Cu2þ ion in industrial wastewater without any sample preparation steps. Thus, the present method would 7
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Designing of glutathione-lactose derivative for the fabrication of gold nanoclusters with red fluorescence: Sensing of Al3þand Cu2þ ions with two different mechanisms Mehul R. Kateshiya a, Naved I. Malek a, Z.V.P. Murthy b, Suresh Kumar Kailasa a, * a b
Applied Chemistry Department, S. V. National Institute of Technology, Surat, 395 007, India Chemical Engineering Department, S. V. National Institute of Technology, Surat, 395 007, India
A R T I C L E I N F O
A B S T R A C T
Keywords: GSH-lactose-AuNCs UV–Visible and fluorescence spectroscopy FT-IR HR-TEM Al3þand Cu2þ ions
In this work, a simple and novel glutathione-lactose (GSH-lactose) derivative was synthesized and used as a template for the fabrication of fluorescent gold nanoclusters (GSH-lactose-AuNCs). The synthesized fluorescent GSH-lactose-AuNCs were stable and showed red emission color by irradiating with 365 nm of UV light. At 410 nm of excitation wavelength, the fluorescent GSH-lactose-AuNCs exhibited a strong emission peak at 635 nm. The fluorescent GSH-lactose-AuNCs acted as a fluorescent sensor for the rapid and sensitive detection of two metal ions i.e., Al3þ and Cu2þ ions with different sensing mechanisms such as fluorescence enhancement for Al3þ ion and fluorescence quenching for Cu2þ ion). As a result, the emission peak of GSH-lactose-AuNCs at 635 nm exhibited independent behaviour i.e., a linear enhancement with the addition of Al3þ ion and a linear quenching efficiency with Cu2þ ion. Thus, good linear relationships were noticed between intensity (I and I0/I) over the concentration ranges of 0.075–100 μM for Al3þ ion and of 0.050–100 μM for Cu2þ ion with correlations co efficients of 0.9962 and 0.9964. The detection limits are 12 and 52 nM for Al3þ and Cu2þ ions. The GSH-lactoseAuNCs-based analytical approach was successfully applied to quantify Al3þ and Cu2þ ions in real samples.
1. Introduction Recently, gold nanoclusters (AuNCs) have received a great deal of scientific attention because of their outstanding applications in applied sciences (sensing, bioimaging, catalysis, therapy, and electronics) [1,2]. AuNCs exhibit unique physicochemical properties that are specifically varied from their large size materials (AuNPs), resulting to possess quantum confinement effects, which is due to the presence of precise Au atoms in each cluster (~150 atoms) and ultrasmall size (<2 nm) [3]. Notably, AuNCs exist with discrete electronic structures that allow them to behave as molecular-like properties, including quantized charging, electronic transitions, molecular chirality and magnetism, respectively [4,5]. Importantly, an one atom difference in AuNCs leads to drastic changes in their optical and physico-chemical properties, revealing their atom- and size-dependent properties [6]. The number of atoms in AuNCs has been controlled by introducing novel ligands as templates for the fabrication of atomically precise AuNCs [7]. Thus, ligand chemistry has been evolved as an emerging research area for understanding the structure and properties of AuNCs and the construction of AuNCs with
novel architecture and desired analytical applications [2]. Ligand chemistry plays a vital role in fabricating atomically precise AuNCs for specific interactions with other chemical species [8–12]. In view of this, various ligands including, proteins, amino acids, egg-white, and small organic molecules have been used as templates for the fabri cation of AuNCs [13–16]. Notably, ligands on the surfaces of AuNCs would offer unique features such as protection of AuNCs, the formation of monodisperse, tuning of surface and optical properties [2]. However, some of the issues (introduction of cost-effective novel ligand, mono dispersity, and high quantum yield) are still unresolved in the chemical approaches for the fabrication of AuNCs using various ligands as pro tecting ligands [17]. Aluminium (Al3þ) has found to be one of the most abundant element in the lithosphere. Alloys of aluminium have been widely used to pro duce distinct structural materials and their uses in various industrial and scientific sectors [18]. Although aluminium alloys and its composites have shown notable advance applications in different areas, exceeding levels of Al3þ ion can cause multiple diseases such as bone softening, breast cancer, Alzheimer’s and Parkinson’s [19,20]. Importantly, a
* Corresponding author. E-mail addresses: [email protected], [email protected] (S.K. Kailasa). https://doi.org/10.1016/j.optmat.2020.109704 Received 11 November 2019; Received in revised form 10 January 2020; Accepted 14 January 2020 0925-3467/© 2020 Elsevier B.V. All rights reserved.
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higher amount of Al3þ ion in the environment is significantly damaged the growth of plants. As per the world health organization, Al3þ ion should be in the range of 7 mg kg/week or 3–10 mg/day for humans [21]. For quantification of Al3þ ion in environments, serval analytical techniques like high-performance liquid chromatography, nuclear ab sorption spectroscopy, atomic absorption spectrometry and mass spec trometry have been adopted to establish analytical procedures for the trace level identification of Al3þ ion [22–26]. Similarly, copper ion (Cu2þ ion) has drawn considerable attention because of its beneficial or dangerous environmental and human health impacts [27]. It has high bio-accumulative nature, causing the inter ruption of cellular balance and serious diseases (Wilson’s and Menke’s diseases) [28]. Various analytical methods have been adopted for Cu2þ ion analysis with elevated selectivity [29,30]. Among the analytical techniques, fluorescence spectrometry has been recognized as one of a cost-effective analytical tool for the analysis of metal ions [31]. For example, Liu’s group described a fluorescence method for identification of Al3þ ion using naphthol aldehyde-tris derivate as a probe via fluo rescence “turn-on” mechanism [32]. Gupta et al. synthesized 1-(2-pyr idylazo)-2-naphthol derivated for selective colorimetric and fluorescence recognization of Al3þ ion [33]. Schiff base receptors have been prepared and used as fluorescent sensors for the analysis of Al3þ ion [34]. A fluorescence-based optical probe was developed for the quantitative analysis of Al3þ ion [35]. Similarly, different fluorescent probes such as DNAzymes, protein, luminescent metal clusters and organic dyes have been reported as promising sensors for assaying of Al3þ and Cu2þ ions [36–38]. It was observed that the assay of Al3þ ion was based on various mechanisms including energy transfer [39,40], charge transfer [41] and chelation enhance fluorescence [35,42]. These adopted procedures have shown to be sensitive methods for the trace level analysis of Al3þ ion, however, some of the methods require highly skilled persons to operate the instruments and inability to on-field monitoring. Furthermore, they required either specific organic ligands or modifications for the selective capturing of Al3þ ion from various environments. In recent years, AuNCs have been elevated as remarkable fluorescent probes for the analysis of various metal ions [43]. For example, Hg2þ, Agþ, Cr3þ, Cr6þ, Zn2þ, Al3þ, Cd2þ and Pb2þ ions have
been recognized by using fluorescent nanoprobes including AuNCs [44–53]. In these methods, various ligands such as glutathione, metal-organic framework, dimercaptosuccinic acid, and cytidine have been used as templates for the preparation of AuNCs. In this work, a novel ligand glutathione-lactose derivative (GSHlactose) was synthesized by the reaction between GSH and lactose via glycosylamine formation (Scheme S1 of Supporting Information). The synthesized GSH-lactose derivative was the first time used as a novel ligand for one-pot synthesis of ultra-small red fluorescent AuNCs (Scheme 1). The GSH-lactose-AuNCs exhibited an intense emission at 635 nm by them exciting at 410 nm. The GSH-lactose-AuNCs showed typical independent behaviour i.e., fluorescence enhancement with Al3þ and quenching with Cu2þ ions, enabling to establish a facile analytical method for parallel detection of Al3þ and Cu2þ ions with lower detection limits. To our knowledge, this ligand is the first report on the synthesis of AuNCs for parallel fluorescence detection of Al3þ and Cu2þ with two different mechanisms. Combining the simplicity in the synthesis of GSHlactose derivative and GSH-lactose-AuNCs, the proposed method has proven to be a gold-standard platform for the detection of Al3þion in the antacid drug tablet and of Cu2þion in copper oxychloride (fungicide) and industrial wastewaters. 2. Experimental section 2.1. Chemicals and materials HAuCl4⋅xH2O, glutathione in the reduced form (GSH) and metal salts were procured from Sigma-Aldrich, USA. Lactose was procured from SRL Chemicals LTD, India. Antacid tablets were purchased from a local medical store, Surat, India. Copper oxychloride 50% WP was purchased from Prakash Sales Agencies, Surat, India. 2.2. Instrumentation The absorption spectra of GSH-lactose-AuNCs were recorded on a Maya Pro 2000 optical spectrophotometer, Ocean Optics, USA. The excitation and emission spectra of GSH-lactose-AuNCs were recorded on
Scheme 1. Schematic represent of GSH-lactose-AuNCs formation with the help of GSH-lactose derivative. 2
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a Cary Eclipse fluorescence spectrometer (Agilent Technologies, USA). The morphology and size of GSH-lactose-AuNCs were confirmed on high-resolution transmission electron microscopy (HR-TEM) (JEM2100, JEOL, Japan). The functional groups of GSH-lactose-AuNCs were recorded by using Fourier transform infrared (FT-IR) spectrometry (FTIR APHA ІІ, Bruker, Germany).
3. Results and discussion 3.1. Spectral characterization of GSH-lactose derivative and GSH-lactoseAuNCs First time, we designed a novel ligand for the synthesis of AuNCs using carbohydrate chemistry and small biomolecule. The bottom-up strategy was carried out for the preparation of GSH-lactose-AuNCs where GSH-lactose derivative acted as a template and reducing agent. The present study provides the following objectives (i) synthesis of GSHlactose derivative with one-step reaction, (ii) controlled growth of AuNCs with fluorescence properties and (iii) development of a facile analytical platform for the detection of trace-level metal ions with two different mechanisms. Briefly, the amino group of glutathione was reacting with the aldehyde group of lactose, yielding yellows lacy product (Scheme S1 of Supporting Information). In this, the gold pre cursor is initially formed the complex with GSH-lactose and then controlled reduction of Au3þ ion to Au0 atom. GSH-lactose provides multi-functional active groups, allowing to generate stable AuNCs with good binding sites of AuNCs surface (Scheme 1). The present design describes that the combination of carbohydrate chemistry and small biomolecule to synthesize novel GSH-lactose derivative for the controlled growth of AuNCs with an average size of 1.03 � 0.37 nm. In order to synthesize GSH-lactose derivative with good yield, GSH-lactose derivative was synthesized by using different concentrations of GSH (0.25, 0.50, 1.0 mmol) and lactose (0.75, 1.5, 3.0 mmol) and studied their absorption spectra (Fig. S1 of Supporting Information). The maximum absorbance peak was noticed at 340 nm using GSH and lactose ratio at 1:3 mmol. Thus, GSH-lactose derivative was synthesized by using a 1:3 mol ratio of GSH and lactose. Fig. 1, displays the optical characteristics (UV–visible, excitation, and emission) of lactose-GSH-Au NCs. In the UV–visible spectrum of GSH-lactose-AuNCs, the absorption maximum was observed at 325 nm, revealing the AuNCs did not show any surface plasmon resonance peak in the visible region, which confirms the generation of fluorescent AuNCs. The GSH-lactose-AuNCs exhibited a remarkable emission peak at 635 nm when excited at a wavelength of 410 nm. In daylight, GSHlactose-AuNCs show a clear yellowish-green color whereas it exhibits a strong red emission under UV light of 365 nm wavelength. To establish optimum conditions for AuNCs, the effect reaction time (1–36 h) and the effect of GSH-lactose amounts (10–50 mg/mL) were investigated on the spectral characteristics of AuNCs (Figs. S2 and S3 of Supporting Infor mation). The maximum emission intensity of GSH-lactose-AuNCs was
2.3. Synthesis of GSH-lactose-AuNCs A mixture of GSH (307.3 mg; 1 mmol) and lactose (1081.01 mg; 3 mmol) with a mole ratio of 1:3 was dissolved in deionized water (2 mL). The mixture was sonicated for 10 min and the mixture was subjected to an oil bath at 130 � C for 30 min. Yellowish lacy product was obtained, suggesting the formation of GSH-lactose derivative. The synthesized GSH-lactose derivative was centrifuge at 12,000 rpm for 15 min, after removing the upper layer of solution, this process 3–4 time repeated. The GSH-lactose derivative was stored for further use in the synthesis of GSH-lactose AuNCs. The GSH-lactose-AuNCs were produced by using a derivative of GSHlactose as an encapsulating agent. In short, 50 mg of GSH-lactose de rivative was dissolved into 1.0 mL of deionized water. The as-prepared GSH-lactose derivative solution (50 mg in 1 ml) was added into a re action flask that contains 5.0 mL of 0.25 M HAuCl4 solution and then the mixture was stirred for 12 h. An equal volume of EtOH (6.0 mL) was mixed with the reaction mixture and then centrifuged at 12,000 rpm for 15 min. The obtained GSH-lactose-AuNCs precipitate was redispersed in Milli-Q water for further characterization and applications. 2.4. Detection of Al3þ and Cu2þ ions To develop a fluorescence analytic approach for assaying of Al3þ and Cu ions, 1.0 mL (0.75 mM) of GSH-lactose-AuNCs was taken into a small glass vial and then added 0.5 mL of various amounts of Al3þ ion (0.075–100 μM) and Cu2þion (0.050–100 μM). The specimens were vortexed for 60 s and kept standing for a few minutes. The emission intensities of solutions were noted by exciting at 410 nm. Further, different common ions (Cd2þ, Mn2þ, Co2þ, Cr3þ, Hg2þ, Ni2þ, Pb2þ, Ca2þ, and Zn2þ,1 mM) were investigated to evaluate the selectivity of the probe. 2þ
2.5. Analysis of Al3þ and Cu2þ ions in real sample GSH-lactose-AuNCs were applied as a probe for the analysis of both Al3þ and Cu2þ ions in real samples (drug, fungicide and industrial wastewater). Antacid drug tablets were purchased from Local Medical Store, Surat, Gujarat, India. The tablets were powdered and 5 mg of tablet powder was taken into a 50 mL standard flask, sonicated for 5 min and then diluted with water up to the mark. The solution was filtered and the amount of Al3þ ion was estimated by using the above procedure. Copper oxychloride is used as a fungicide in many vegetables, and it contains 50% (w/w) of copper oxychloride. To quantify Cu2þ ion in fungicide, 5 mg of copper oxychloride was dissolved in 50 mL of deionized water with a slight addition of dilute NaOH. From this, different concentrations of copper oxychloride (50, 100, 200 nM) were prepared and estimated Cu2þ ion concentration as per the described procedure earlier. To evaluate the practical application of the method, the amount of Cu2þ ion was estimated in industrial wastewater, Chamuda Brass Products Pvt, Ltd, Jamnagar, Gujarat, India. The collected wastewater was filtered with Whatman filter paper to remove the microsize parti cles. The Cu2þ ion was quantified in industrial wastewater as per the described procedure in the above section.
Fig. 1. UV–visible spectra of GSH-lactose, fluorescence excitation (at 410 nm) and emission (at 635 nm) spectra of GSH-lactose-AuNCs (Inset: the image of GSH-lactose-AuNCs at under UV lam at 365 nm and daylight). 3
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GSH at 8.6 ppm was disappeared in the 1H NMR spectrum of GSHlactose derivative, confirming the formation of GSH-lactose derivative. Fig. S5a of Supporting Information shows that the FT-IR spectral characteristics of pure GSH. The characteristic GSH absorption bands around 1713-1613 cm 1 belong to –COO– group (symmetric stretching). The peaks at 1396 and 1713 cm 1 are generated due to –COO– (asym – O (stretching). The peak of –OH of COOH metric stretching) and C– group is noticed at 1279 cm 1 and the two peaks at 3350-3027 cm 1 are corresponded to -N-H stretching (symmetric and asymmetric). The characteristic –SH group stretching of GSH was appeared at 2524 cm 1. Fig. S5b of Supporting Information displaces that the FT-IR spectrum of pure lactose. A broad peak was observed at 3528-3100 cm 1 which is due to poly hydroxyl (-OH) groups of lactose. The peaks at 2900 and 1657 cm 1 belong to–CH- group stretching and -C-O- six members cyclic ring. The GSH-lactose derivative FT-IR spectrum was shown in
noticed at 635 nm using reaction time at 12 h and 50 mg/mL of GSHlactose. Thus, GSH-lactose-AuNCs are synthesized using 50 mg/mL of GSH-lactose at 12 h as reaction time. To determine functional groups of GSH-lactose derivative, 1H NMR and FT-IR spectra were studied. Fig. S4a of Supporting Information shows that the 1H NMR of pure GSH, the peak at 8.6 ppm corresponds to the protons of –NH2 and the peak at 8.4 ppm belongs to the protons –NH–. The peaks at 4.4–3.7 ppm are attributed to –CH– protons of GSH. The peaks at 2.8–2.2 ppm correspond to –CH2 protons. Fig. S4b of Supporting Information shows that the 1H NMR of pure lactose. The peaks at 6.3–5.1 and 2.6 ppm belong to the –CH– protons of lactose. The peaks at 4.8–3.6 ppm correspond to protons of –OH of lactose. Fig. S4c of Supporting Information shows that the 1H NMR of GSH-lactose deriva tive. It can be clearly noticed that the peaks of –NH- group of GSH was appeared at 8.0–5.6 ppm, however the characteristic NH2 group peak of
Fig. 2. HR-TEM images of GSH-lactose-AuNCs at (a) 2 nm scale bar (b) lattice fringes of GSH-lactose-AuNC at 2 nm scale bars, (c) histogram of GSH-lactose-AuNCs drawn at 1 nm scale bar. HR-TEM images of GSH-lactose-AuNCs with (d) Al3þ ion and (e) Cu2þ ion. Histograms drawn at 1 nm scale for GSH-lactose-AuNCs with (f) Al3þ ion and (g) Cu2þ ion. 4
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Supporting Information Fig. S5c. The broad peak at 3351-3293 cm 1 belongs to–NH and –OH (stretching) groups. The C–H stretching was observed at 2931 cm 1. The peaks at 2524, 1673 and 1078 cm 1 are generated due to –SH and -C-O- stretchings of six member ring and–NH bending. Fig. S5a of Supporting Information shows that the FT-IR spectrum of GSH-lactose-AuNCs. The characteristic functional groups (-COOH, –NH2, and –OH) of GSH-lactose are either slightly shifted or disappeared due to the formation of GSH-lactose-AuNCs. Noticeably, the stretching peak of –SH group of GSH was completely disappeared due to Au3þ ion-GSH-lactose complex formation. To complement the above spectroscopic characterizations and to estimate the actual size of GSH-lactose-AuNCs, HR-TEM was studied to examine the size and morphology of GSH-lactose-AuNCs (Fig. 2a–c). Fig. 2b shows that the lattice fringes (d ¼ 0.12 nm) spacing is matched with the face-centred cubic (fcc) with (111), demonstrating the crys talline nature of GSH-lactose-AuNCs. The as-synthesized GSH-lactoseAuNCs exhibited ultrasmall size dispersity with an average size of 1.03 � 0.37 nm (Fig. 2c), favouring molecular-like absorption characteristics, which allow them to act as ideal probes for molecular recognition. The quantum yield (QY) of GSH-lactose-AuNCs was estimated with the help of Rhodamine B as a reference and the QY of GSH-lactose-AuNCs is found be 74.48%. The as-synthesized GSH-lactose-AuNCs was very good stability up to 195 days (Fig. 6S of Supporting Information).
investigated on the fluorescence emission intensity of GSH-lactoseAuNCs in the presence of both metal ions (Al3þ ion - 500 μM and Cu2þ ion - 500 μM) (Fig. S7 of Supporting Information). It was noticed that the fluorescence emission intensity of GSH-lactose-AuNCs was gradually increased with increasing time from 0.5 to 7.0 min and then reached to plateau at 4.0 min. Similarly, at 1.0 min, the fluorescence emission intensity of GSH-lactose was maximum quenched with the addition of Cu2þ ion. Thus, 4.0 and 0.5 min were selected as optimum time for the measuring the fluorescence emission spectra of GSH-lactoseAuNCs with Al3þ and Cu3þ ions. To explain possible sensing mechanism of the method, we carried out HR-TEM studies of GSH-lactose-AuNCs in presence of Al3þ and Cu2þ ions (Fig. 2d–g). In HR-TEM of GSH-lactose-AuNCs, the average sizes of GSH-lactose-AuNCs were increased from 1.03 � 0.37 to 6.45 � 0.28 and 5.05 � 0.18 nm with the addition of Al3þ and Cu2þ ions, demonstrating the aggregation of GSH-lactose-AuNCs. Furthermore, the fluorescence quenching of GSH-lactose-AuNCs by Cu2þion was also evaluated using the Stern-Volmer equation [15]. I0/I ¼ 1 þ Ksv [Q] Where I0 and I fluorescence intensities of AuNCs with and without Cu2þ ion. Ksv is the Stern-Volmer quenching constant and [Q] is the concen tration of Cu2þ ion. The Stern-Volmer equation shows good linearity with increasing concentration of Cu2þ ion (Fig. S8b of Supporting In formation), revealing the dynamic or static quenching in the fluores cence emission of AuNCs with Cu2þ ion. To confirm further, timecorrelated single-photon counting was studied by using the fluores cence lifetime of GSH-lactose-AuNCs with and without Cu2þ and Al3þions (Fig. S9 of Supporting Information). The GSH-lactose-AuNCs exhibited an average lifetime as 11.70 ns, however it is decreased to 10.60 ns with the addition of Cu2þ ion. These life-time spectra revealed that the formation of a complex between GSH-lactose-AuNCs and Cu2þion, which results to quench the emission of AuNCs. Similarly, the average lifetime of GSH-lactose-AuNCs was increased to 14.68 ns with the addition of Al3þ ion, illustrating the sensing mechanism for Al3þ ion is based on the aggregation-induced emission.
3.2. Analysis of Al3þ and Cu2þ ions and sensing mechanism The fluorescence emission behaviour of GSH-lactose-AuNCs was evaluated by the addition of various trace metal ions. Fig. 3 shows that the emission spectrum of GSH-lactose-AuNCs at the excitation wave length of 410 nm. After being independent treatment with various metal species (Cu2þ, Al3þ, Cd2þ, Mn2þ, Co2þ, Cr3þ, Hg2þ, Ni2þ, Pb2þ, Ca2þ, and Zn2þ, 1 mM), the maximum emission peak at 635 nm underwent a remarkable enhancement for Al3þ ion and a drastic quenching for Cu2þ ion, whereas other metal ions did not induce any obvious fluorescence enhancement or quenching. Thus, GSH-lactose-AuNCs could be acted as a promising probe for assaying of Al3þ and Cu2þ ions by using fluores cence spectrometry. To expand the sensing ability of GSH-lactose-AuNCs probe for detection of Al3þ and Cu2þions, the effect of reaction time was
3.3. pH study for assaying of Al3þ and Cu2þ ions The pH of the solution has a great impact on the fluorescence behaviour of GSH-lactose AuNCs. The pH of the solution is a very important factor for the detection of target analytes. In view of this, the emission peak intensity of GSH-lactose-AuNCs was investigated in the absence and presence of both ions (Al3þ and Cu2þ) as a function of so dium acetate and phosphate buffer saline (PBS) pH (2.0–12.0). Initially, the fluorescence intensity of GSH-lactose-Au NCs was studied without addition of metal ions (Al3þ and Cu2þ ion) at sodium acetate and PBS pH range from 2.0 to 12.0 (Figs. S10–S11 of Supporting Information). The fluorescence emission intensity of GSH-lactose-Au NCs was drastically changed at both sodium acetate and PBS pHs 2.0 and 12.0. It can be clearly noticed that at sodium acetate and PBS pHs 2.0 and 12.0, the fluorescence intensity was either quenched or enhanced without addi tion of both metal ions. These results demonstrated that these pHs may not be considered as optimum pHs for the detection of both ions. Since, at high acidic pH, active functional groups were protonated, resulting strong charge repulsions and hydrogen bonding, which yields to enhance emission intensity without addition of Al3þ ion. Figs. S10–S11 of Supporting Information exhibit the fluorescence response by the addition of Al3þ and Cu2þ ions at sodium acetate and PBS pHs 2.0–12.0. For Al3þ ion, the fluorescence intensity of GSH-lactose-AuNCs was enhanced maximum at sodium acetate buffer pH 9, showing higher in tensity as compared to the emission spectra at remaining pH (Fig. S10b of Supporting Information). Therefore, an Al3þ ion assay was carried out at sodium acetate buffer pH 9.0 using GSH-lactose-AuNCs as a probe. Similarly, the fluorescence spectrum of GSH-lactose-AuNCs was
Fig. 3. Fluorescence spectra of GSH-lactose-AuNCs after addition of different metal ions (Cu2þ, Al3þ, Cd2þ, Mn2þ, Co2þ, Cr3þ, Hg2þ, Ni2þ, Pb2þ, Ca2þ, and Zn2þ, 1 mM), and (inset: photograph under UV lamp at 365 nm after addition of metal ions). 5
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maximum quenched with Cu2þ ion at PBS pH 9.0 (Fig. S11 of Supporting Information), suggesting the PBS pH 9.0 is optimum pH for assay of Cu2þ ion. The sodium acetate pH 9.0 and PBS pH 9.0 were chosen as optimum pH for the detection of Al3þ and Cu2þ ions using GSH-lactose-AuNCs as a probe.
ion. The limits of detection are 12 and 52 nM for Al3þ and Cu2þ ions, exhibiting much better sensitivity than the reported methods in the literature [54–58] (Table 1). 3.5. Interference study The selectivity of GSH-lactose-AuNCs towards Al3þ and Cu3þ ions was carried out using different chemical species (Fig. 5a and b). Under optimal conditions, the mixture of metal ions (Mg2þ, Cd2þ, Hg2þ, Fe2þ, Ni2þ, Ba2þ, Ca2þ, Cr3þ, Zn2þ, Fe3þ and Pb2þ, 1 mM), anions (Cr2O27 , PO34 , SO24, S2 , F , Cl , Br , and I , 1 mM) and pesticides (chlorprop ham, difenoconazole, fipronil, tebuconazole, metalaxyl, isoproturon and chlorpyrifos, 1 mM) had shown no significant effect on the fluorescence behaviour of GSH-lactose-AuNCs (Fig. 5a and b), however, the fluores cence intensity of GSH-lactose-AuNCs was enhanced with Al3þ ion and quenched with Cu2þ ion even at strong interferents chemical species. These spectral data clearly demonstrated that GSH-lactose-AuNCs acted as an optical sensor for both ions (Al3þ and Cu2þ) in the presence of other competitive chemical species, demonstrating its selectivity for assaying of Al3þ and Cu2þ ions.
3.4. Sensitivity study for Al3þ and Cu2þions To explore the sensitivity of the probe, Al3þ and Cu2þ ions with different concentrations were added independently into the GSHlactose-AuNCs at sodium acetate pH 9.0 and PBS pH 9.0, and their resulting spectra were recorded (Fig. 4a and b) for the calibration graphs construction. For Al3þ and Cu2þ ions: the calibration graphs were con structed by using various concentrations of Al3þ ion (0.075–100 μM) and of Cu2þ ion (0.050–100 μM) (Fig. S8 of Supporting Information). These results indicate that the fluorescence emission of GSH-lactoseAuNCs at 635 nm increases as Al3þ ion concentration raising, and the degree of quenching increases as Cu2þ ion concentration increases. The linear functions of the calibration curves are I ¼ 11.704xþ36.879 (Al3þ ion) with a correlation coefficient (R2) of 0.9962 for Al3þ ion and I0/I ¼ 1.1115xþ0.9847 with a correlation coefficient (R2) of 0.9964 for Cu2þ
3.6. Practical applications 3.6.1. Detection of Al3þion in antacid tablets and industrial wastewater The detection of Al3þ ion in the antacid drug was carried out by using GSH-lactose-Au NCs as a probe. Briefly, each tablet (600 mg) consists 250 mg of Al(OH)3 and Mg (OH)2, 50 mg (active dimethicone) and 50 mg (magnesium aluminium silicate hydrate). Firstly, the standard so lutions of the antacid tablet were prepared by dissolving 50 mg of tablet and then added 1–2 drops of dilute HCl. Then, 3 different concentrations (0.05, 0.06 and 0.07 μg/mL equal to 170, 270 and 330 nM) of the above solution were added into 1 mL of GSH-lactose-AuNCs solution (Fig. S12 of Supporting Information). The amount of Al3þ ion was quantified by the described procedure earlier and results were shown in Table S1 of Supporting Information. Similarly, GSH-lactose-AuNCs was used as a simple optical probe for the analysis of Al3þion in industrial wastewater, which was collected from the manufacturing of Aluminium boxes and bottles in Jamnagar, India. The collected industrial wastewater was filtrated by using Whatman filter paper and then 3 different dilution sample taken and directly added into GSH-lactose-AuNCs solution (Fig. S13 of Supporting Information). The Al3þ ion concentration is found to be 80.23, 100.54 and 210.68 nM and shown in Table S1 of Supporting Information. 3.6.2. Detection of Cu2þion in fungicide (copper oxychloride) and industrial wastewater Copper oxychloride is widely used as a fungicide to control fungi cides in many vegetables and it contains 50% (w/w) of copper oxy chloride. To explore the application of the method, copper oxychloride (50, 100 and 200 nM) solutions were spiked on vegetables (tomato, Table 1 Analytical merits of the present method for fluorescence assays of Al3þ and Cu2þ ion as compared with reported methods in the literature.
Fig. 4. Fluorescence emission spectra of (a) GSH-lactose-AuNCs with different concentration of Al3þ (0.075–100 μM) (inset: photograph under UV lamp at 365 nm after addition of Al3þ (0.075–100 μM)) and (b) GSH-lactose-AuNCs with different concentration of Cu2þ (0.050–100 μM) (inset: photograph under UV lamp at 365 nm after addition of Cu2þ (0.050–100 μM)). 6
Probe
Metal ion
Linear range (μM)
Detection limit (μM)
Reference
Citrate-Au NPs Rhodamine based fluorescent chemosensors Chemosensor Dithiothreitol capped Au NCs Glutathione protected Au NCs GSH-lactose-Au NCs
Al3þ Al3þ
0.5–20
7.4 0.12
[54] [55]
Al3þ Cu2þ
0–60
1.5 80
[56] [57]
Cu2þ
0.1–6.25
86
[58]
Al3þ and Cu2þ
0.075–1 and 0.050–1
0.011 and 0.052
Present work
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allow a facile, fast and reliable analytical method for detection and screening of Al3þ and Cu2þ ions from various samples (food packing materials, tablets, and industrial wastewater). 4. Conclusions In summary, a novel GSH-lactose derivative was designed as a novel organic ligand for the fabrication of fluorescent AuNCs. The GSHlactose-AuNCs acted as a probe for the detection of trace-level detec tion of Al3þ ion via fluorescence enhancement and of Cu2þ ion via fluorescence turn-off mechanisms. The developed probe displayed wider linear ranges of 0.075–100 μM for Al3þ ion and of 0.050–100 μM for Cu2þ ion. The method exhibited lower detection limits, and the probe was successfully applied to assay Al3þ and Cu2þ ion in various samples including food packing material, tablet, fungicide, and industrial wastewater, respectively. Thus, this method could be used as a prom ising analytical method for the quantitative analysis of Al3þ and Cu2þ ions in various samples. Author contributions section Mehul R.Kateshiya, Writing – original draft, He has carried out entire experimental work and written the experimental sections of manuscript, Naved I. Malek, Data curation, He is involved in the discussion of FT-IR spectral data, Z.V.P.Murthy, Data curation, He corrected grammatical mistakes and discussed about TEM data, Suresh Kumar Kailasa, Super vision, Completely supervised the work. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by the Department of Science and Technology, Government of India (EMR/2016/002621/IPC). MK acknowledges the Director, SVNIT, Surat, India for the doctoral fellowship to carry this work.
Fig. 5. Fluorescence emission spectra of GSH-lactose -Au NCs with (a) Al3þ ion upon the addition of various chemical species; inorganic species (metal ions Mg2þ, Cd2þ, Hg2þ, Fe2þ, Ni2þ, Ba2þ, Ca2þ, Cr3þ, Zn2þ, Pb2þ and Fe3þ, 1.0 mM; anions - Cr2O27 , PO34 , SO24, S2 , F , Cl , Br , and I , 1.0 mM) and pesticides (chlorpropham, difenoconazole, fipronil, tebuconazole, metalaxyl, isoproturon and chlorpyrifos, 1.0 mM) inset: photograph under UV lamp at 365 nm and (b) Cu2þion upon the addition of various chemical species; inorganic species (metal ions - Mg2þ, Cd2þ, Ni2þ, Hg2þ, Fe2þ, Ba2þ, Ca2þ, Pb2þ, Cr3þ, Zn2þ and Fe3þ ion, 1.0 mM; anions - Cr2O27 , PO34 , SO24 , S2 , F , Cl , Br , and I , 1.0 mM) and pesticides (chlorpropham, difenoconazole, fipronil, tebuconazole, metalaxyl, isoproturon and chlorpyrifos, 1.0 mM) (Inset: photograph under UV lamp at 365 nm).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.optmat.2020.109704. References [1] R. Jin, C. Zeng, M. Zhou, Y. Chen, Atomically precise colloidal metal nanoclusters and nanoparticles: fundamentals and opportunities, Chem. Rev. 116 (2016) 10346–10413. [2] R.R. Nasaruddin, T. Chen, N. Yan, J. Xie, Roles of thiolate ligands in the synthesis, properties and catalytic application of gold nanoclusters, Coord. Chem. Rev. 368 (2018) 60–79. [3] N. Goswami, Q. Yao, T. Chen, J. Xie, Mechanistic exploration and controlled synthesis of precise thiolate-gold nanoclusters, Coord. Chem. Rev. 329 (2016) 1–15. [4] S. Knoppe, O.A. Wong, S. Malola, H. Hakkinen, T. Burgi, T. Verbiest, C.J. Ackerson, Chiral phase transfer and enantioenrichment of thiolate-protected Au102 clusters, J. Am. Chem. Soc. 136 (2014) 4129–4132. [5] M.A. Aljuhani, M.S. Bootharaju, L. Sinatra, J.-M. Basset, O.F. Mohammed, O. M. Bakr, Synthesis and optical properties of a dithiolate/phosphine-protected Au28 nanocluster, J. Phys. Chem. C 121 (2017) 10681–10685. [6] J. Fang, J. Li, B. Zhang, X. Yuan, H. Asakura, T. Tanaka, K. Teramura, J. Xie, N. Yan, The support effect on the size and catalytic activity of thiolated Au25 nanoclusters as precatalysts, Nanoscale 7 (2015) 6325–6333. [7] C. Yao, J. Chen, M.-B. Li, L. Liu, J. Yang, Z. Wu, Adding two active silver atoms on Au25 nanoparticle, Nano Lett. 15 (2015) 1281–1287. [8] X. Dou, X. Yuan, Q. Yao, Z. Luo, K. Zheng, J. Xie, Facile synthesis of water-soluble Au25-x Agx nanoclusters protected by mono-and bi-thiolate ligands, Chem. Commun. 50 (2014) 7459–7462.
cabbage, and brinjal) and the amount of Cu2þ ion was estimated as per the illustrated procedure earlier. It was noticed that the present method exhibited a good relative standard deviation from 0.21 to 0.63% (Table S2 of Supporting Information). Further, the potential application of the present method was described for the detection of Cu2þ ion in industrial wastewater, Cha muda Brass Products Pvt, Ltd, Jamnagar, India. The collected waste water sample was filtered by Whatman filter paper to remove dust and then the 3 different dilution sample taken and directly added to the GSHlactose-AuNCs solution, the concentration of Cu2þ ion in industrial wastewater was estimated by the described procedure earlier and the data were shown in Table S2 of Supporting Information). The obtained fluorescence spectra were shown in Fig. S14 of Supporting Information. This method was successfully detected Cu2þ ion in industrial wastewater without any sample preparation steps. Thus, the present method would 7
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