SEARCH

SEARCH BY CITATION

Keywords:

  • antibiotics;
  • fluorescence;
  • gold;
  • nanoparticles;
  • quantum clusters

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusion
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

The syntheses of gold nanoparticles (Au NPs) and gold quantum clusters (Au QCs) that employ cefadroxil (CFX; an antibiotic) as both reducing and capping agents are reported. The HAuCl4/CFX concentration, temperature, and pH are crucial factors in the modulation of the nucleation and growth kinetics of the reaction, and consequently, in guiding the size and morphology of as-synthesized Au NPs. Interesting results are observed if the reaction is performed under different pH conditions. TEM analysis of the Au NPs synthesized at pH 6 shows an average particle size of approximately 2 nm along with a relatively smaller population of bigger NPs (up to 6 nm). The Au QCs were isolated by high-speed centrifugation and showed fluorescence at λ≈460 nm. Furthermore, the as-synthesized Au QCs were applied as sensor for Sn4+ ions on the basis of an aggregation-induced fluorescence quenching mechanism. These Au QCs offer acceptable sensitivity, high selectivity, and a limit of detection of approximately 10 μM for the determination of Sn4+ ions.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusion
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

In recent decades, nanosized materials have become a topic of enormous concern because their tendency to acquire noble properties at this length scale depends on the alteration in size, shape, and most importantly, the material itself.1 Major classes of nanomaterials, such as semiconductor nanoparticles (NPs), ferromagnetic/electric NPs,25 polymeric particles, carbon-based nanostructures,69 and metallic NPs (MNPs), have various individual applications in biological labeling, sensing, and multiple color imaging owing to size-dependent fluorescence,1013 cell-sorting,1416 magnetic resonance imaging (MRI),17, 18 drug delivery,1921 magnetic hyperthermia therapy,22, 23 and increased drug solubility.2428 In comparison to other systems, MNPs are the most alterable nanostructures because of better synthetic control of their size, shape, structure, assembly, encapsulation, and accompanied tuneability of their electronic properties.24

Among the MNPs, gold nanoparticles (Au NPs) are most promising for biomedical applications because of their simple and fast synthesis; excellent biocompatibility using appropriate ligands; bioconjugation; shape-, size-, and surrounding chemical-environment-dependent tuneable optoelectronic properties, suitability as a platform for multifunctionalization with a wide range of organic or biological ligands.2933 Recently, among various biomedical issues, the resistance of bacteria to cephalosporin antibiotics has been established as an issue of big concern, owing to the enhancement of resistant strains.34 These bacterial strains develop an enzyme called β-lactamase, which has the ability to break the β-lactam ring of cephalosporin antibiotics.35, 36 Therefore, efforts have been undertaken that include modification to the β-lactam ring37 and the development of highly efficient new antibiotics38 to combat the resistant bacterial strains. However, all of these approaches are either time consuming or economically not feasible. Antimicrobial formulations in the form of NPs act as an effective bactericidal material against Gram-positive and Gram-negative bacteria.3943 In the literature, numerous studies are available in which people have shown the antimicrobial effects of antibiotic-conjugated NPs.4143 However, most of them used a lengthy and time-consuming multistep procedure that included NPs synthesis, functionalization, and binding of antibiotics onto the functionalized NP surface.

Therefore, for the first time, our group has made an effort to simplify the above-mentioned prolonged procedure into a one-step method by using the antibiotic cephalexin itself as a reducing and capping agent for the synthesis of Au NPs.44 In another study, we performed in situ observations of the unusual nucleation and growth kinetics of Au NP synthesis in the presence of cephalexin using in situ static and dynamic light scattering.45 We have noticed that two well-separated populations of gold particles, with sizes of 1–2 nm versus several tens of nanometers, respectively, nucleate simultaneously.45 In the same direction, Rai et al. described a one-pot synthesis of spherical Au NPs of various sizes at a range of temperatures (20 to 70 °C) with cefaclor and demonstrated potent antimicrobial activity.46

However, there are few untouched aspects remaining from the previous studies, such as 1) a comprehensive study that includes the effect of variation in pH of the reaction on the synthesis of antibiotic-conjugated Au NPs; and 2) the synthesis, isolation, and characterization of antibiotic-labeled gold particles with size <2 nm (known as quantum clusters (QCs)). The idea for the synthesis of Au QCs came from our study,45 in which we observed the simultaneous synthesis of two different types of Au NP populations with size of 1–2 nm versus several tens of nanometers. Similar results were reported by Zheng et al.47 for their synthetic procedure, in which they performed centrifugation to settle out relatively large NPs and used the colorless supernatant, which contained Au QCs, for further characterization. They reported the absence of a surface plasmon resonance (SPR) peak (λ≈520 nm) in the UV/Vis spectrum, which indicated the presence of particles with sizes of <2 nm.47 Nowadays, Au QCs have become the subject of huge curiosity because they bridge the properties of isolated atoms and NPs.48 QCs show molecular properties and discrete energy states with core-size-dependent fluorescence, which follows the relationship EFermi/n1/3 (Jellium model).48

Herein, we describe the use of cefadroxil (CFX) antibiotic to synthesize Au NPs and Au QCs, in which CFX acts as both a reducing and surface capping agent. CFX differs from cephalexin through an extra [BOND]OH group on the benzene ring. We chose CFX because, at equivalent oral doses, it has a longer serum half-life, slower urinary excretion rate, and greater area under the serum level versus time curve than that of cephalexin or cephradine. Therefore, CFX has greater persistence in serum and urine, and more prolonged in vivo bacterial exposure.49

The as-prepared Au QCs at pH 6, followed by centrifugation, exhibited fluorescence emission at λ≈460 nm. Subsequently, we used these Au QCs as a sensing probe for Sn4+ ions based on a mechanism that involved particle aggregation-induced fluorescence quenching. This water-soluble sensing probe offered high selectivity in discriminating Sn4+ ions from other metal ions as well as high sensitivity. Tin has been widely used for over hundred years as a robust form of food packaging material in the manufacturing of beverage cans. The use of tinplate for food and beverage packaging results in some tin dissolving into the food content, particularly if plain uncoated internal surfaces are used. From published data, there is an association between the consumption of food containing tin at concentrations up to 200 ppm and significant acute adverse gastrointestinal effects.50 Considering these facts, it is beneficial to detect the presence of Sn4+ ions in the environment at very low concentrations, especially in the micromolar range.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusion
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

UV/Vis–NIR spectroscopic and TEM studies

To visualize the effect of HAuCl4/CFX concentration, temperature, and pH on the optical properties, size, and shape of as-synthesized Au NPs, detailed UV/Vis–near-infrared (NIR) spectroscopic and TEM studies were performed.

Effect of HAuCl4/CFX concentration

As mentioned in the Experimental Section, an aqueous solution of CFX (final concentration in the mixture was 5×10−4M) was added to an aqueous solution of HAuCl4 (final concentration in the mixture was varied from 10−4 to 10−3M) with continuous stirring at 25 °C for 2 h followed by UV/Vis–NIR spectroscopic and TEM studies. We have shown the effect of changes in concentration on the UV/Vis–NIR absorption spectra in Figure 1. When the final concentration of HAuCl4 in the mixture was 10−4M, a single, broad peak was observed at λ≈530 nm, corresponding to transverse-mode surface plasmon resonance (TSPR), and indicated the presence of isotropic NPs in the suspension. As we increased the final concentration of HAuCl4 from 10−4 to 5×10−4M, a new peak started to appear in the NIR region at λ≈730 nm (Figure 1), corresponding to the longitudinal-mode SPR (LSPR), which was due to increased departure from an isotropic spherical structure toward an anisotropic structure.4446 Further increase in the HAuCl4 concentration from 5×10−4 to 10−3M leads to a shift in the position of the LSPR peak from λ≈730 to 1120 nm with no significant change in the position of the TSPR peak. The redshift in the LSPR peak with increasing concentration of HAuCl4 is symptomatic of increases in the edge length of the anisotropic nanostructures.51

thumbnail image

Figure 1. The UV/Vis–NIR spectra of the CFX-labeled Au NPs (CGNPs) synthesized at various final concentrations of HAuCl4 (10−4M to 10−3M) with a fixed final concentration of CFX of 5×10−4M.

Download figure to PowerPoint

The effect of variation in the relative concentration and their signature on the surface plasmon excitation peaks was also confirmed by TEM analysis. The TEM images in Figure 2 a, b, and c show the CGNPs synthesized at final concentrations of HAuCl4 of 10−4, 5×10−4, and 10−3M, respectively, with a fixed final concentration of CFX of 5×10−4M. These images reconfirm that, at a HAuCl4 concentration of 10−4M, isotropic Au NPs with an average size of about 7 nm form (Figure 2 a), which is consistent with the optical absorbance data discussed earlier. On the other hand, for a HAuCl4 concentration of 5×10−4M, the presence of small triangular NPs (along with spherical NPs) with an average size of about 36 nm can be seen (Figure 2 b). Furthermore, if the concentration of HAuCl4 was increased to 10−3M, the edge length of the triangular NPs increased (from about 40 to 90 nm); this was consistent with the UV/Vis data. The presence of anisotropic structures could be due to 1) retardation of growth along the <111> direction because of preferential chemisorption of Cl ions on the (111) lattice planes, 2) a multiply twinned seed-mediated mechanism, and 3) room-temperature (25 °C) annealing of irregular, nanoparticulate aggregates.5254

thumbnail image

Figure 2. TEM images of CGNPs synthesized at final concentrations of a) 10−4, b) 5×10−4, and c) 10−3M of HAuCl4, with a fixed final concentration of CFX of 5×10−4M.

Download figure to PowerPoint

Effect of temperature

In addition to HAuCl4/CFX concentration, temperature is also an important parameter for the modulation of the nucleation and growth kinetics of Au NPs synthesis. Therefore, we investigated the role of reaction temperature by mixing the pre-thermalized solutions (at the specific temperature mentioned herein) of CFX and HAuCl4 (final concentration in the mixture of 5×10−4M) at a temperature range from 10 to 50 °C, as depicted in Figure 3.

thumbnail image

Figure 3. The UV/Vis–NIR spectra for the CGNPs synthesized at various temperatures (at a final concentration of 5×10−4M for both HAuCl4 and CFX) from 10 to 50 °C.

Download figure to PowerPoint

At a reaction temperature of 50 °C, a single peak at λ≈570 nm (corresponding to TSPR) was obtained, which indicated the synthesis of isotropic NPs. If the reaction temperature was decreased to 40 °C, a new peak started to appear at λ≈855 nm (corresponding to LSPR), which showed the synthesis of anisotropic NPs. Further decreases in the reaction temperature to 30 and 20 °C led to the LSPR peak shifting to longer wavelengths (λ≈876 and 997 nm, respectively), which again indicates an increase in the edge length of the anisotropic nanostructures (Figure 3).51 Interestingly, if the reaction was performed at 10 °C, the strong and broad absorption in the whole NIR region (λ≈1176 nm) indicated the presence of an anisotropic structure with high aspect ratio (along with isotropic nanostructures) in the suspension.

The effect of temperature can also be seen from the corresponding TEM images given in Figure 4. Figure 4 a–d shows the TEM images of CGNPs synthesized at 50, 40, 20, and 10 °C, respectively. The TEM image in Figure 4 a shows the synthesis of nearly isotropic NPs at 50 °C. At 40 °C, the formation of anisotropic NPs, such as triangles and rods, began, as seen in the Figure 4 b. Upon further decreasing the reaction temperature to 20 °C, an increase in the lengths of the rods and edges of the nanotriangles was seen; this was consistent with UV/Vis results. The TEM image in the Figure 4 d shows the synthesis of long rods with lengths of up to 250 nm (with an average particle size of ca. 52 nm) at 10 °C. This can be understood by the inter-relationship of two processes 1) particle growth rate and 2) CFX adsorption rate.55, 56 To facilitate the growth of anisotropic NPs, CFX must be adsorbed onto the unstable facet of the intermediate before spherical particles are formed. It is a relatively slow process and can occur at low temperature at which 1) the rate of reduction of metal ions becomes slow and possibly facilitates the oriented growth of nuclei, and thus, promotes the formation of anisotropic NPs; and 2) the decreased growth rate of gold particles leads to the intermediates remaining in solution for a relatively long time, and thus, the probability of CFX being adsorbed onto unstable facets of the gold intermediate increases. Increasing the temperature during the reaction results in an increase in the rate of reduction of gold ions; this in turn leads to an enhanced nucleation rate and most of the chloroaurate ions are consumed in the formation of nuclei, and thus, the secondary reduction process on the surface of the preformed nuclei is stalled.51 In other words, as the temperature increases, the growth rate of gold increases, and the intermediate rapidly disappears. Consequently, we obtain a much higher population of spherical NPs in comparison with triangular ones. Thus, simple variation in the temperature of the reaction enables synthesis and the tailoring of the size of the triangular NPs.

thumbnail image

Figure 4. TEM images of CGNPs synthesized at a) 50, b) 40, c) 20, and d) 10 °C with fixed final concentrations of CFX and HAuCl4 of 5×10−4M.

Download figure to PowerPoint

Effect of pH

We also studied the effect of pH on the morphology of Au NPs by mixing an aqueous solution of CFX with an aqueous solution of HAuCl4 at final concentration of 5×10−4M, while changing the pH from 4 to 8 (the pH of the solution was fixed by using different pH buffers). We observed drastic changes in the UV/Vis–NIR spectra, as seen in Figure 5.

thumbnail image

Figure 5. UV/Vis–NIR spectra of CGNPs synthesized under various pH conditions (pH 4–8; final concentration of 5×10−4M for both HAuCl4 and CFX). The inset shows different colored solutions of CGNPs synthesized at various pH values.

Download figure to PowerPoint

A comparison of absorption spectra at pH 4 and 5 shows that the SPR peak position blueshifts from λ≈545 to 535 nm and the color of the suspension changes from intense red to dark grey, which indicates a decrease in the size of the nanospheres. If the reaction was performed at pH 6, the color of the suspension changed to brown and the SPR peak diminished to a large extent, which indicated the synthesis of very small particles with sizes of <2 nm (known as Au QCs).47, 48 In contrast, a new peak appeared in the UV range centered at λ≈350 nm. Furthermore, at pH 7 and 8, we observed that the color of the reaction suspension remained unchanged, with absorption maxima at λ≈357 and 351 nm, respectively (Figure 5). The probable reason of the UV absorbance in the about 350 nm region could be the synthesis of antibiotic–gold complexes.57, 58

To study the change in the size and shape of as-synthesized Au NPs, TEM analysis was performed (Figure 6). Figure 6 a, b, and c shows TEM images corresponding to gold particles synthesized at pH 4, 5, and 6, respectively. TEM images corresponding to pH 7 and 8 are not shown because we were unable to find Au NPs in these samples. If the reaction was performed at pH 4, isotropic particles with an average size about 10 nm were synthesized (Figure 6 a). The average particle size decreased to about 6 nm if the reaction was performed at pH 5; furthermore, at pH 6, the average particle size decreased to about 2 nm, along with a relatively smaller population of bigger NPs (up to 6 nm). Our study clearly demonstrates that the pH of the reaction medium is also a crucial factor in the modulation of reaction kinetics for the synthesis of Au NPs. Au NPs form through the reduction of Au3+ to Au0 and CFX is expected to have this ability for the synthesis of Au NPs by being oxidized in this process. It is anticipated that this ability would depend on the degree of ionization of different groups present in the CFX molecules. Furthermore, the degree of ionizability of different groups depends on the pH of the reaction medium. Therefore, in conclusion, the ability to synthesize Au NPs by employing CFX molecules depends on the pH of reaction mixture to a large extent.

thumbnail image

Figure 6. TEM images of CGNPs synthesized at a) pH 4, b) pH 5, and c) pH 6 with fixed final concentrations of CFX and HAuCl4 of 5×10−4M.

Download figure to PowerPoint

The crystalline nature of the as-synthesized CGNPs was studied through powder XRD (PXRD; Figure S1 in the Supporting Information), which revealed the dominance of the (111) plane in the anisotropic Au NPs. Moreover, FTIR, Raman, and XPS studies have been performed to analyze the functional group responsible for the binding of CFX to the surface of the Au NPs. FTIR and Raman spectroscopic results (Figures S2 and S3 in the Supporting Information, respectively) indicate the possible involvement of [BOND]NH2 and >C[DOUBLE BOND]O groups in the binding of CFX to the surface of the Au NPs.

XPS study

For further chemical analysis of CGNPs, we compared the XPS spectra of CFX with CGNPs for the binding energy (B.E.) of C 1 s, N 1 s, O 1 s, and S 2 p core-level electrons for CFX alone (Figure 7 a, c, e, and g) and CGNPs (Figure 7 b, d, f, h, and i). As-obtained XPS core-level spectra were background-corrected by using the Shirley algorithm, and chemically distinct peaks were resolved by using a nonlinear least-squares fitting procedure. The core-level binding energies were aligned with the carbon B.E. of 285 eV. It is evident from Figure 7 a that the C 1 s core-level spectrum of CFX could be resolved into four components situated at approximately 283.2, 285, 286.5, and 288.4 eV. The peak at 285 eV is due to electrons from saturated hydrocarbons, whereas the peaks at 286.5 and 288.4 eV can be assigned to the [BOND]COOH group and α-carbon bound to [BOND]COOH and [BOND]NH2 groups present in the CFX molecule, respectively.44 The peak with the smallest intensity, situated at 283.2 eV, is assigned to carbon present in the aromatic ring. However, the C 1 s core-level spectrum of CGNPs could be resolved into three components situated at approximately 285, 286.5, and 288.4 eV B.E.,44 as presented in Figure 7 b. In Figure 7 c, we have plotted the N 1 s core-level spectrum of neat CFX, which has two peaks at approximately 400.3 and 401.8 eV and may be due to the two different nitrogen atoms in the CFX molecule with different electronic environments59 (for detailed assignments, see Table S1 in the Supporting Information). These peaks were shifted to 401.9 and 403.4 eV in the N 1 s core-level spectrum of CGNPs. We observed a significant shift (ca. 1.6 eV) in the B.E. of the N 1 s core-level spectrum of CGNPs; thus we concluded that the nitrogen moiety might have been responsible for binding with the Au NPs in this case. These results obtained from the N 1 s core level spectrum are consistent with our Raman spectroscopic data.

thumbnail image

Figure 7. XPS data for 5×10−4M CFX (a, c, e, and g) and CGNPs (b, d, f, h, and i) synthesized at final concentrations of 5×10−4M for both HAuCl4 and CFX.

Download figure to PowerPoint

Figure 7 e and f shows the spectra of the O 1 s core level (for CFX and CGNPs, respectively), which is at 532 eV for both molecules;60 this indicates that, even after the reaction, the electronic environment of oxygen atoms in the CFX molecule are unchanged. The S 2 p core-level spectra are given in Figure 7 g and h. In Figure 7 g (for CFX), the main peak at a B.E. of 164.7 eV may be analyzed as two peaks situated at 164.3 and 165.6 eV and assigned to 2 p3/2 and 2 p1/2, respectively. On the other hand, in Figure 7 h (for the CGNPs), the peak corresponding to the S 2 p core level is situated at 163 and 167 eV and could be resolved into four peaks at 162.7, 164, 166.8, and 168.3 eV. We assigned the peaks at approximately 162.7 (S 2 p3/2) and 164 eV (S 2 p1/2) to free sulfur, that is, not bound to the Au NPs.61 Further, the remaining signals at approximately 166.8 (S 2 p3/2) and 168.3 eV (S 2 p1/2) were assigned to sulfur, which was bound to the Au NPs (S[BOND]Au). The large shift towards higher B.E. for sulfur shows possible oxidation at this site after the synthesis of Au NPs;44 this indicates that sulfur might play a significant role in the reduction/binding process. In Figure 7 i, we have presented the Au 4 f core-level spectrum, which shows two peaks situated at 84.3 and 87.9 eV that are attributed to 4 f7/2 and 4 f5/2, respectively, from the Au0 state.44 The peak obtained at higher a B.E. of approximately 87.9 eV with the lowest intensity is probably due to unreduced gold ions. It is well documented that amine derivatives6266 and thiol groups6769 strongly bind to the gold surface and our XPS results also clearly indicate that the sulfur and amine groups of the CFX molecules play a vital role in binding to the surface of the Au NPs.

Fluorescence probe for the detection of Sn4+ ions

As discussed in the Experimental Section, to isolate Au QCs, the pH 6 reaction suspension was centrifuged at 15 000 rpm for 15 min. In comparison to the UV/Vis spectrum of an aqueous solution of CFX (Figure 8 a), isolated Au QCs show a continuous absorption in the UV range, starting from λ≈490 nm, as shown in Figure 8 b; in addition, the complete absence of the SPR absorption peak at λ≈520 nm confirms the presence of Au QCs with a size of <2 nm.47, 48 The corresponding HR-TEM image is presented in Figure 9 and confirms the absence of large NPs. These Au QCs exhibit fluorescence excitation and emission maxima at λ=360 and 460 nm, respectively (Figure 8 c and d).

thumbnail image

Figure 8. Normalized absorbance spectrum of aqueous solutions of a) CFX and b) CFX-labeled Au QCs; c) fluorescence excitation spectrum; and d) emission spectrum of CFX-labeled Au QCs.

Download figure to PowerPoint

thumbnail image

Figure 9. A HR-TEM image of CFX-labeled Au QCs. The inset shows the HR-TEM image of aggregated Au QCs after treatment with a solution of Sn4+ ions.

Download figure to PowerPoint

Furthermore, the suspension of Au QCs was directly used as a highly sensitive and selective fluorescence “turn-off” sensor for Sn4+ ions without any modification. It is inferred from Figure 10 that the fluorescence of CFX-labeled Au QCs was effectively quenched by Sn4+ ions. As indicated in Figure 10, the fluorescence quenching of Au QCs is highly selective for Sn4+ ions. Upon interaction with different bi-, tri-, and tetravalent ions (200 μM), the fluorescence intensity of CFX-labeled Au QCs shows a substantial decrease for Sn4+ ions, whereas the fluorescence intensity of Au QCs is nearly unaffected by metal ions other than Sn4+.

thumbnail image

Figure 10. a) Quenching of the fluorescence of CFX-labeled Au QCs by Sn4+ ions. b) Selectivity and c) sensitivity of Au QCs toward Sn4+ ions (the concentration of all metal ions was 200 μM; F0 and F1 correspond to the fluorescence intensity of Au QCs in the absence and presence of metal ions, respectively).

Download figure to PowerPoint

Figure 10 b shows the selectivity of Au QCs as Sn4+ ion sensor. The dose response graph was employed to determine the relationship between the fluorescence intensity of the Au QCs and the concentration of the quenching ion Sn4+, that is, the dependence of the quenching effect (F0/F1) on the concentration of the quencher. The fitted linear data could be expressed by Equation (1).(1)

  • equation image(1)

The limit of detection (LOD) for Sn4+ ions was about 10 μM and the slope was 0.00533 μM−1. The plot shows a linear relationship (R2=0.97955) between the fluorescence decrease with the concentration of Sn4+ ions over the range from 10 to 160 μM (Figure 10 c).

The fluorescence quenching of samples of CFX-labeled Au QCs in the presence of Sn4+ ions was attributed to the aggregation of Au QCs, which might be induced by CFX and Sn4+ ion complexation. Figure 9 shows the HR-TEM image of Au QCs before and after treatment with Sn4+ ions and confirms the aggregation of Au QCs. Unlike most organic fluorophores, the high water dispersibility of fluorescent Au QCs allows this sensor to be used in aqueous media without the need for organic cosolvents.

Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusion
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

We have described a facile one-pot method, using CFX as a reducing/capping agent, for the kinetically modulated synthesis of Au NPs by employing changes in the major reaction parameters, such as HAuCl4/CFX concentration, temperature, and pH. The synthesis of triangular Au NPs was achieved by increasing the HAuCl4/CFX concentration, whereas the synthesis of rods was achieved upon decreasing the temperature of the reaction to 10 °C. Interesting results were observed if the reaction was performed under different pH conditions. At pH 6, we obtained a highly stable, brown-colored suspension with an average particle size of approximately 2 nm. Furthermore, our FTIR, Raman, and XPS results confirmed the binding of CFX to the surface of Au NPs.

With a short reaction time and easy purification, for the first time, this strategy allows the fabrication of Au QCs with the capability to sense Sn4+ ions. On the basis of an aggregation-induced fluorescence quenching mechanism, these Au QCs offer acceptable sensitivity, high selectivity, and a LOD about 10 μM for the determination of Sn4+ ions. These Au QCs may be used for various environmental and biological applications.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusion
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Materials

All chemicals were of analytical grade and used as received without any further purification, unless otherwise described. HAuCl43 H2O (≥49.0 % Au basis), La(NO3)36 H2O (99.9 %), Dy(NO3)3xH2O (99.9 %), CuSO45 H2O (≥98.0 %), dibasic sodium phosphate (>99 %), monobasic sodium phosphate (>99 %), CH3COOK (>99 %), sodium acetate (≥99 %), and acetic acid (>99.7 %) were purchased from Sigma Aldrich. CFX was purchased from Fluka. (NH4)2Ce(NO3) (99.9 %) and Zn(NO3)26 H2O (96–103 %) were purchased from Loba Chemie. MgCl2 (≥98 %), Na2CO3 (99.9 %), and Ba(NO3)2 (>99 %) were purchased from Merck. Cr(NO3)39 H2O (98–101 %) and SnCl45 H2O (97.5 %) were purchased from Thomas Baker. An aqueous solution of 10−2M HAuCl4 and 10−2M CFX was used as a stock solution and refrigerated at 4 °C. All glassware was washed with aqua regia (HCl/HNO3=3:1) carefully and rinsed with Millipore water (18.2 MΩ resistance) before being used in the reaction.

Synthesis of CFX-labeled Au NPs

CGNPs were synthesized through CFX-mediated reduction of HAuCl4. The absence of toxic materials in the synthetic procedure makes CGNPs highly promising candidates for bio-nanomedicines. Herein, a detailed study was carried out on the mineralization behavior of CFX for the synthesis of Au NPs in the presence of various reaction conditions, such as concentration, temperature, and pH. CGNPs were synthesized by the addition of an aqueous solution of CFX (final concentration in the mixture of 5×10−4M) to an aqueous solution of HAuCl4 (final concentration in mixture was varied from 10−4M to 10−3M) followed by continuous stirring for 2 h at 25 °C. The effect of temperature (10 to 50 °C) and pH (4 to 8) on the synthesis of Au NPs was also studied in detail at a final concentration of 5×10−4M for both CFX and HAuCl4 in the mixture.

Isolation of CFX-labeled Au QCs

During the course of the reaction performed at pH 6, the simultaneous production of Au NPs and Au QCs in the reduction of HAuCl4 with CFX occurred and the color of the solution changed from light yellow to dark brown. Au QCs were probably generated by the modulated reduction of HAuCl4 by CFX at pH 6. Relatively large particles were removed at the end of reaction through centrifugation at 15 000 rpm for 15 min to provide a light-brown supernatant that contained CFX-labeled Au QCs.

Fluorescence detection of Sn4+ ions by using CFX-labeled Au QCs

We developed a fluorescence quenching based turn-off sensor for Sn4+ ions by using CFX-labeled Au QCs. We investigated the effect of adding various metal ions (200 μM) on the fluorescence of CFX-labeled Au QCs. An aliquot of Au QCs (50 μL) was added to an aqueous solution (2 mL) of various concentrations of Sn4+ ions. The solution was mixed thoroughly and left to react at room temperature for 5 min. The fluorescence emission spectra were then recorded at an excitation wavelength of 360 nm.

Characterization techniques

UV/Vis–NIR and photoluminescence spectroscopy were conducted to analyze the optical properties of the CGNPs. Absorbance spectra were recorded by using a Jasco UV/Vis–NIR dual-beam spectrometer (Model V570) operated at a resolution of 2 nm. Fluorescence excitation and emission spectra were recorded by using a Cary Eclipse photoluminescence spectrophotometer from Varian equipped with a xenon flash lamp. The particle morphology of the CGNPs was characterized by TEM. TEM images were recorded by using a Tecnai F20 TEM instrument from FEI equipped with a field-emission source operating at 200 kV. The sample was drop-casted on a carbon-coated copper grid and air dried in vacuum before being introduced into the TEM instrument. HR-TEM measurements on CFX-labeled Au QCs were performed by using a Tecnai F30 HR-TEM instrument from FEI equipped with a field-emission source operating at 300 kV with a S-TWIN objective lens and Cs value of 1.2 mm. The point resolution of the microscope was 0.24 nm. PXRD patterns were obtained to confirm the synthesis of crystalline phases in CGNPs. PXRD patterns were recorded by using a PHILIPS X′PERT PRO instrument equipped with X′celerator, a fast solid-state detector, on drop-coated samples on a glass substrate. The samples were scanned by using X′celerator with a total number of active channels of 121. Iron-filtered CuKα radiation (λ=1.5406 Å) was used. The XRD patterns were recorded in the 2θ range of 20–80°.

Binding studies of CFX on the surface of Au NPs were performed by FTIR (Fourier transform infrared), XPS (X-ray photoelectron spectroscopy), and Raman spectroscopy. FTIR spectra were obtained by using a PerkinElmer Spectrum One instrument. The spectrometer operated in the transmission mode at a resolution of 4 cm−1. The samples for FTIR studies were mixing with KBr powder and allowed to dry and the dried powder was directly used for FTIR studies. Raman spectroscopy measurements were recorded at room temperature on a HR 800 Raman spectrophotometer (Jobin Yvon, Horiba, France) by using monochromatic radiation emitted by a He–Ne laser (633 nm) operating at 20 mW. The experiment was repeated several times to verify the consistency of the recorded spectra. The samples for the Raman spectroscopy studies were prepared simply by drop-casting the liquid onto a glass slide. XPS analysis was performed on a VG Micro Tech ESCA 3000 instrument at a pressure of <1×10−9 Torr with an overall resolution of 0.1 eV. The spectra were recorded with monochromatic AlKα radiation at a pass energy of 50 eV and an electron take-off angle of 60°. The sample was drop-casted on a clean Si wafer and air dried for analysis.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusion
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

P.P. acknowledges support from a Young Scientist Award grant from the Council for Scientific and Industrial Research (CSIR) in Physical Sciences and a separate grant from the Department of Science and Technology (DST), India (DST/INT/ISR/P-8/2011). P.P. also acknowledges Dr. G. V. Pavan Kumar, Indian Institute of Science Education & Research (IISER), Pune, India, for providing the use of a Raman spectrometer. P.K., D.K.S., and S.S. acknowledge the DBT for the award of a Junior Research Fellowship, the CSIR for the award of a Nehru Post Doctorate Fellowship in Science, and the CSIR for the award of a Senior Research Fellowship, respectively.

Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusion
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

FilenameFormatSizeDescription
cplu_201300319_sm_miscellaneous_information.pdf685Kmiscellaneous_information

Please note: Wiley Blackwell is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.