Yanlin Yuab,
Lei Yan
*a and
Zhining Xiab
aSchool of Criminal Investigation, Southwest University of Political Science and Law, Forensic Science Engineering Research Center of Universities in Chongqing, Chongqing 401120, China. E-mail: yanlei@swupl.edu.cn
bSchool of Bioengineering, Chongqing University, Chongqing 400044, China
First published on 27th October 2017
Non-toxic Au nanoclusters@montmorillonite (AuNCs@MMT) nanocomposites with strong red fluorescence are prepared by a microwave (MW)-assisted synthesis method and immobilize Au NCs into sodium MMT clay matrix via electrostatic interaction. Due to the immobilization of Au NCs in the layered structure of the MMT clay matrix, the formative AuNCs@MMT nanocomposites show high emission, stable chemical features and less toxicity. The obtained Au NCs and AuNCs@MMT nanopowders were characterized by using UV-visible absorption spectroscopy, fluorescence spectroscopy, infrared spectroscopy, TEM/HRTEM, SEM and XRD etc. to depict their sizes, microstructures and optical features. Due to their environmentally friendly preparation, time-saving procedure, user-friendly operation, low cost, efficient UV-visible radiation-dependent photoluminescence and good affinity with finger residues, the as-synthesized AuNCs@MMT nanopowders are employed as a alternative florescent developing reagent for enhancing latent fingerprints deposited on various object surfaces (such as glass, porcelain enamel, stainless steel, painted metal, plastic products, weighing papers etc.) for individual identification. As results, the enhanced fingerprints with clear patterns and satisfactory ridge details were obtained by using as-prepared AuNCs@MMT nanopowders. At the same time, intensively red fluorescence as well as good contrast without background staining of developed prints demonstrated great advantages for surfaces with multicolour. Because of their good sensitivity, non-toxicity and strong resistance to background interference, the as-prepared fluorescent nanocomposites are an actual alternative to conventional powdering reagents, which may find potential application in forensic detection.
Montmorillonite (MMT) is a very soft phyllosilicate group of minerals. Low-cost, environmentally friendly and its porous characteristics make it attract more and more attentions. And it was used as a matrix material to form kinds of complex substances with different properties. CdTe–montmorillonite nanocomposites were synthesized by in situ intercalation of CdTe quantum dots (QDs) into a sodium montmorillonite clay.21 The products with different emissions were used as developing powder enhanced latent fingermarks, and the developed prints were well-defined in the terms of ridge details. However, the third-level of ridge characteristics (sweat pores) were not presented in their images. In addition, CdTe QDs are known as a highly poisonous material, the toxicity of the powder cannot be ignored, which will increase the risk of harm to the operators.
Here, we demonstrate for the first time that AuNCs@MMT nanocomposites can be prepared via electrostatic interaction and the resulting powders show strong red fluorescence under UV-visible light irradiation. The procedure is emerging as a rapid, effective and environmentally friendly method for preparing fluorescent nanocomposites. The main attribute of this work is that our red emitting AuNCs@MMT nanocomposites could be conveniently synthesized in a few minutes without complicated experimental processing and conditions as well as toxic chemicals and solvents. The synthetic conditions including the applied MW programme and the concentrations of Au NCs are reported. The Au NCs and AuNCs@MMT nanocomposites are well characterized by various spectroscopic techniques and electron microscopes. Finally, the potential use of AuNCs@MMT nanocomposites for developing latent fingermarks is explored. The obtained fluorescent powders of AuNCs@MMT nanocomposites may find potential application in forensic science for individual identification.
All chemicals of analytical reagent grade were used without further purification. Ultrapure Millipore water (18.2 Ω) was used throughout all the experiments.
In a typical adsorption experiment, 2.0 mL as-synthesized Au NCs solution was added into 0.2500 g Na+-MMT powder. After ten minutes-magnetic stirring, the precipitate were separated by centrifugation and dried at 35 °C under vacuum condition. The products displayed an intensive red fluorescence emission under UV light (365 nm), indicating the formation of AuNCs@MMT nanocomposites. The as-prepared Au NCs solution and AuNCs@MMT powders were characterized by UV-visible absorption spectroscopy, fluorescence spectroscopy, infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), transmission electron microscope/high-resolution transmission electron microscope (TEM/HRTEM) and X-ray diffraction (XRD).
Au NCs with intensively red photoluminescence were synthesized by using microwave irradiation method. By the environment-friendly and high-efficiency method, a dark brown solution of Au NCs was prepared which would be employed as a fluorescent probe to prepare developing reagent for latent fingerprints detections. In our previous studies, it was found that excessive heat from the MW irradiation would reduce quantum yield of fluorescent AuNCs.22 Consequently, a MW programme consisting of 15 seconds MW irradiation, 30 seconds pause, and 15 seconds MW irradiation was applied to subsequent MW-assisted preparation of fluorescent Au NCs. Compared with our previous methods, a teflon high-pressure digestion tank is employed as the reaction vessel to replace beaker in this study, which is beneficial to form high temperature and pressure in such a short span of tens seconds. Hence, the reaction time can be shortened from several minutes to a few seconds, thanks to the super-heating and high pressure. The reaction mixture turns from light yellow to dark brown after MW irradiation (inset of Fig. S1a†) which is consistent with our previous work.22 Similarly, the solution of products emit an intense red fluorescence (inset of Fig. S1a†) under UV light (λ = 365 nm) as well as green light. The formation of these fluorescent particles is attributed to the in situ reduction of Au3+ to Au0 embedded in the BSA molecules. Subsequently, the aggregated Au atoms form minor sized Au NCs of highly fluorescent.28 And a broad emission band (550–750 nm) with an emission peak maximum of 640 nm is observed. The maximum excitation wavelength is 525 nm, which is green light. Compare to UV light, green light owns lower energy, can excite less substrates, which also means it could reduce background interferences. Furthermore, take green light as an alternatives of UV light will decrease the health risk of examiners. Fig. S1b† displays the UV-visible absorption spectrum of reactants and the Au NCs product. An absorption peak of 530 nm demonstrates the formation of Au NPs, which do not appear in the spectra of BSA and HAuCl4.
Solid form of the as-synthesized Au NCs was obtained by adjusting pH value of solution from alkalinity to acidity.22 After vacuum drying, the powders of Au NCs also possess very strong red emission under UV light and green light irradiation. As we know, main constitute of the nanopowders is BSA, which is easily to form crystalline solid. Therefore, the dispersion degree and grain size of as-synthesized Au NCs powder is not very satisfactory for latent fingerprint detection. In order to improve the property of gold nanopowders, Na+-MMT powder, a natural clay with porous structure, was chosen as the matrix for immobilization Au NCs. The procedure of preparation AuNCs@MMT nanocomposites is depended on electrostatic interaction between negatively charged Au NCs and positively charged sodium ion, which are dispersed in aqueous solution and modified into the inner surfaces of MMT clay, respectively. In order to confirm above-mentioned deduction, zeta potential of the as-prepared Au NCs was measured. The zeta potential of Au NCs solution was determined to be −28.4 mV at pH 10, demonstrating that the Au NCs can be easily attracted to positively charged sodium ions by electrostatic interaction. At the same time, it also indicates that the Au NCs solution has good stability which is beneficial for its storage, preservation and application. Consequently, the same as Au NCs, as-prepared powders of AuNCs@MMT nanocomposites emit an intense fluorescence under UV light (λ = 365 nm) and green light.
Besides the size of Au NCs, its proportion in the AuNCs@MMT product is the other main factor on the performance of latent fingerprint staining. The higher load amount of AuNCs immobilized into MMT, would produce better sensitivity and resolution. In this experiment, ten minutes-magnetic stirring was used to assist the electrostatic adsorption process, and we found that the color of Au NCs solution got lighter and no longer changes, which means the adsorption of Au NCs on MMT has been saturated.
Fig. 1a depicts the fluorescence excitation and emission spectra of AuNCs@MMT nanocomposites powders prepared under optimized conditions. Compared with no fluorescent clay powders, as-prepared AuNCs@MMT nanocomposites powders owns strong red emission (Fig. 1a and inset of Fig. 1b). In the range of 600–700 nm, a finger-like emission spectrum with an emission peak maximum of 655 nm is observed. The maximum excitation wavelength (λex) is at 525 nm which is in agreement with the solution of Au NCs and was selected for the subsequent fluorescence measurements. In summary, these results demonstrate that fluorescent Au NCs endue MMT clay the red emission, makes the AuNCs@MMT nanocomposites could be supposed as a fluorescent probe for fingerprinting. And quantum yield of the as-prepared AuNCs@MMT nanocomposites powders were measured to be 1.16%. Although this data is below the value of Au NCs solution, strikingly red fluorescence still cause enough contrast for the detection of latent fingermarks. Fig. 1b depicts absorption spectra of bare MMT clay powders and AuNCs@MMT nanocomposites powders. No significant absorption peaks and changes were detected, which means small amount of Au NCs that is immobilized to the MMT clay does not change the UV absorption characteristics of the MMT clay. The inset of Fig. 1b shows the photographic images of AuNCs@MMT nanocomposites taken under broad sunlight and UV light (λ = 365 nm). As can be seen, the powders of Au NCs@MMT nanocomposites own good dispersion and emit an intense red fluorescence under UV light (365 nm).
The morphology of BSA–Au NCs bioconjugates, bare MMT clay powder and as-prepared AuNCs@MMT nanocomposites were studied by using TEM/HRTEM and SEM. Fig. S2† displays the HRTEM and TEM images of BSA–Au NCs bioconjugates. The TEM and HRTEM images indicate that the average diameter of Au NCs is ca. 4 nm and NCs are well-dispersed, which is consistent with the particle size test results (the average diameter is 4.085 nm). The lattice fringes of the Au NCs are consistent with metallic Au having a discerned lattice spacing of 2.3 Å, corresponding to the d spacing of the {111} crystal plane of Au, which is consistent with the literature.29 The selected area electron diffraction (SAED) pattern also indicates that the well organized Au nanocrystals are formed in the Au NCs. SEM image of bare MMT clay powders indicates that micron-sized near-spherical particles with lamellate structure provide more surfaces for immobilization of Au NCs. The layered structure is beneficial to increase the loaded amount of Au NCs, which is also means the fluorescent intensity of AuNCs@MMT powders would be improved. Consequently, high sensitivity of latent fingerprints detection could be obtained. Furthermore, compare to micron-sized MMT clay powder, nano-sized Au NCs almost invisible, so there is no obvious image of Au NCs been observed from SEM images (Fig. 2). At the same time, due to extremely low relative content, no signal of gold was determined in the EDX measurement.
In order to obtain additional information regarding the AuNCs@MMT nanocomposites, XPS, XRD and IR of the as-prepared powders were conducted and the results are displayed in Fig. 3. The binding energy (BE) of the Au 4f5/2 and Au 4f7/2 peaks is 84.0 and 87.7 eV (Fig. 3a and b), respectively, and the ΔBE is 3.7 eV, inferring that elemental Au NCs are entrapped in the layered structure of MMT clay. The phase structure of bare MMT clay and the AuNCs@MMT fluorescent nanocomposites were characterized by XRD measurements. The corresponding XRD patterns, shown in Fig. 3c displays exactly the same spectra, which indicates immobilization of Au NCs will not change the phase structure of MMT clay powders. Furthermore, Fig. 3d depicts the IR spectra of bare MMT clay (bottom), BSA (middle) and the as-prepared AuNCs@MMT nanocomposites (top). From our previous study, we know that the formation of Au NCs bring little changes for the IR spectrum of BSA, inferring that the Au NCs embedded in BSA would not affect the surface-structure of BSA.22 Nevertheless, compare with IR spectrum of bare MMT clay, distinct changes were observed in the IR spectrum of AuNCs@MMT nanocomposites. More details of infrared characteristic bands and absorption peaks were obtained, which are come from BSA. The results confirm the presence of BSA-stabilized Au NCs, which also means fluorescent Au NCs are successfully embedded in MMT clay.
Multi-coloured surfaces often own sophisticated patterns and different shades. It is difficult to obtain clear ridges and good contrast for latent fingerprints on theses surfaces by using only one kind of conventional powders, no matter dark powders or light powders. In order to solve this problem, fluorescent powders based on some kind of pigments or dyes have been employed as ingredients of developing reagents. At the same time, UV light is usually employed as the excitation light source with the use of fluorescent reagents. Compare with visible lights, UV light possess higher energy as we know it, which also can excite background fluorescent materials. If both developing powder and background materials have the same or approximate fluorescent emitted wavelengths, it is not easy to distinguish developed prints. As-prepared AuNCs@MMT nanocomposite is a newly-developed fluorescent powder which differentiates from most conventional fluorescent developing reagents, it not only can be excite by UV light but also visible light (green light).
Fig. 5 displays the images of developed fingerprints on the bottom of a ceramic mug, which were excited by sunlight and UV light (λ = 365 nm), respectively. Despite its light colour under sunlight provide an extremely weak pattern impression for backdrop it showed intensively red fluorescence under UV light (365 nm). Under sunlight condition, it is almost impossible to distinguish developed ridges from the yellow-based multicolour background. However, take UV light as an alternative excitation light source, clear ridges patterns and minutiae points with highly red fluorescence could be obtained, relatively dark background provided good contrast, hardly any interference was recorded. In order to observe more details of the ridge, a document imaging workstation was employed. As can be seen from Fig. 5c, the second-level of ridge characteristics, such as bonding point, bifurcation and ridge termination, can be recognized, those are useful in matching a fingerprint to a specific person. And the number of these second-level characteristics is more than twenty, which is enough to guarantee the accuracy of identification. Consequently, taking visible light as the excitation light source could avoid common background fluorescence interferences. As such, it should be attractive for further applications in latent fingerprints visualization. At the same time, use visible light replace UV light would avoid UV damage for the substrates and examiners. Furthermore, the visualization process is accomplished by powder technique, due to avoid contact with organic solvents and dyes, this fluorescent powder reduced the harm for the operators.
In order to evaluate its stability under normal site inspection working conditions, AuNCs@MMT nanocomposites powder was prepared and placed in an ordinary plastic box for seven months, which experienced more than 60 days of sustained high temperature over 30 °C (in July and August). Then it was been used as a developing powder for enhancement of a fresh sebaceous latent fingerprints deposited on plastic surface (Blue-Ray CD discs). As can be seen from Fig. 6, developed prints exhibits clear ridges and minutiae points, even for the third-level of ridge characteristics (sweat pores). The result suggested that the as-prepared AuNCs@MMT nanocomposites powder owns good stability, and its highly red emission cause enough contrast for latent fingerprints visualizing, which is applicable to on-site investigation works. Comparison with semiconductor quantum dots in resolution, the as-prepared Au NCs@MMT nanocomposites powder owns considerable developing ability (such as in resolution, sensitivity and contrast), or better. Moreover, the powder technique is easier to preserve and operate than the solution method.
The right image of Fig. 6 presents the well-defined boundary between ridge and valley areas of the fingerprint. By comparing magnified images of ridge detail with valley regions, we can confirm directly that the majority of AuNCs@MMT nanocomposites powder bind to the ridges with little amounts bound to valley areas. The results indicated that powders adhered to the fingerprint residues. The principle is the same as the conventional powder technique, that is the electrostatic interactions between the powder and moisture or/and oily components of the fingerprint residues.30
In this study, no toxicity assessment of the as-prepared AuNCs@MMT nanocomposites was performed. The presumed non-toxicity of AuNCs@MMT nanocomposites was only based on studies found in the literatures.31–35 It is shown that Au NCs exhibits no increased risk, unlike the cadmium based quantum dots and organic dyes, which could release free toxic cadmium ions and volatilize toxic organic compounds, respectively. Au NCs can be used as biocompatible probe for cellular imaging,31,32 tumour imaging and deep tissue therapy.33 Moreover, MMT clay is a very soft phyllosilicate group of minerals, which is effective as an adsorptive of heavy metals as well as toxic substance, and also can be used as a medicine to treat contact dermatitis for external use.34,35 Despite these encouraging deductions, related toxicological studies are yet to be conducted and the actual risks inherent to the use of nanoparticles remain difficult to ascertain. Therefore, it is recommended to work in a well-ventilated room or in the direction of the wind, and wear adequate personal protective equipment.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra09444b |
This journal is © The Royal Society of Chemistry 2017 |