Rapid visualization of latent fingermarks using gold seed-mediated enhancement
© The Author(s) 2016
Received: 1 October 2016
Accepted: 13 November 2016
Published: 25 November 2016
Fingermarks are one of the most important and useful forms of physical evidence in forensic investigations. However, latent fingermarks are not directly visible, but can be visualized due to the presence of other residues (such as inorganic salts, proteins, polypeptides, enzymes and human metabolites) which can be detected or recognized through various strategies. Convenient and rapid techniques are still needed to provide obvious contrast between the background and the fingermark ridges and to then visualize latent fingermark with a high degree of selectivity and sensitivity.
In this work, lysozyme-binding aptamer-conjugated Au nanoparticles (NPs) are used to recognize and target lysozyme in the fingermark ridges, and Au+-complex solution is used as a growth agent to reduce Au+ from Au+ to Au0 on the surface of the Au NPs. Distinct fingermark patterns were visualized on a range of professional forensic within 3 min; the resulting images could be observed by the naked eye without background interference. The entire processes from fingermark collection to visualization only entails two steps and can be completed in less than 10 min. The proposed method provides cost and time savings over current fingermark visualization methods.
We report a simple, inexpensive, and fast method for the rapid visualization of latent fingermarks on the non-porous substrates using Au seed-mediated enhancement. Au seed-mediated enhancement is used to achieve the rapid visualization of latent fingermarks on non-porous substrates by the naked eye without the use of expensive or sophisticated instruments. The proposed approach offers faster detection and visualization of latent fingermarks than existing methods. The proposed method is expected to increase detection efficiency for latent fingermarks and reduce time requirements and costs for forensic investigations.
Fingermarks are deposited when the ridged skin surface of a finger touches an object and creates an imprint on the object’s surface [1, 2]. The unique patterns of fingermarks make them become one a key form of physical evidence in forensic investigations [2–4]. When imprinted on opaque media such as paint, fingermarks are directly visible to the naked eye. However, latent fingermarks are not visible to the naked eye and can only be detected through visualizing certain residues (such as amino acids and lipids) present within the fingermark [5–7]. The rapid and reliable visualization of latent fingermarks is important to help police quickly identify potential suspects by comparing crime-scene fingermarks against existing fingermark databases. Currently common methods include the use of carbon powder, cyanoacrylate or triketohydrindene hydrate. Carbon powder physically adsorbs the latent fingermarks, while cyanoacrylate and triketohydrindene hydrate react with the protein’s primary amine to visualize latent fingermarks. Recently, various optical, chemical and physical techniques based on nanomaterials have been developed to provide obvious contrast between the background and the fingerprint ridges [8–10].
Recently, studies of specific biomolecule-targeting and nanomaterials have raised the possibility of increasing selectivity and sensitivity in fingermark [11–13]. Wood et al. reported a highly selective technique using a lysozyme targeting-aptamer-based reagent to visualize latent fingermarks with fluorescence images . Shan et al.  and Xu et al.  respectively combined electrochemistry with surface plasmon resonance (SPR) and enzyme immunoassay to detect fingermarks. Li et al. used the SPR of aptamer-tagged Au nanoparticles (NPs) to visualize fingermarks [17, 18]. Very recently, He et al. developed a simple method termed immunological multimetal deposition (iMMD) which combines immunoassay and conventional MMD to allow for the naked-eye visualization of sweat fingermarks using silver staining . The iMMD method requires fewer steps and less time than conventional and improved MMD. However, background interference can negatively impact the visual enhancement of the fingermark ridges during the silver staining processes. In practice, the concentrations and incubation times of silver staining are difficult to calculate and control because of the unknown quantity of Au NPs in the fingermarks. For the silver staining solution, increased incubation time will produce an obvious background, thus reducing the visual contrast of the ridges.
Preparation of tetrakis(hydroxypropyl)phosphonium chloride-stabilized Au (THPC-Au) NPs
THPC-Au NPs were prepared as previously described . First, the THPC solution was prepared by diluting 12 µL of 80% THPC solution to 1 mL with deionized water. Subsequently, 1 mL of the as-prepared THPC solution, 0.5 mL of NaOH (1 M), and 45 mL of deionized water were vigorously stirred in a flask at room temperature for 5 min. 10 mL of HAuCl4 (5 mM) was then added. Once the color of the solution turned from light yellow to dark brown, indicating the formation of THPC-Au NPs, the mixture was stirred for another 2 h. THPC-Au NP solution was stored at 4 °C for further use. The average size of the as-prepared THPC-Au NPs was calculated using transmission electron microscopy (TEM, H-7500; Hitachi Koki Co., Tokyo, Japan). Electron micrographs of THPC-Au NPs and LBA-Au NPs were obtained by placing a drop of the sample onto a copper mesh coated with an amorphous carbon film and dried in a vacuum desiccator.
The particle concentration of THPC-Au NPs was based on the Au ion concentration measured using inductively-coupled plasma (ICP) analysis to calculate as µM. Au ion concentrations were measured by ICP in terms of mg/L and the concentration could be converted to mmole/L. The average size of THPC-Au NPs could then be determined via TEM imaging. We assumed Au NPs were spherical structures to calculate the volume of a single Au sphere, and the volume of a single Au atom could be calculated based on its diameter. Dividing the volume of one Au NP by the volume of one Au atom obtains the number of Au atoms contained in a single Au NP. We assumed that N Au atoms could form one Au NP. Therefore, the particle concentration (µM) of THPC-Au NPs could be calculated after dividing the Au ion concentration (µM) by N. The particle concentration of as-prepared THPC-Au NPs was 2.64 µM.
Preparation of lysozyme-binding aptamer (LBA)-conjugated THPC-Au (LBA-Au) NPs
Thiol (SH)-modified LBAs (HS-LBA, 5′-HS-TTTTTTATCAGGGCTAAAGAGTGCAGAGTTACTTAG-3′) were used to prepare LBA-Au NPs. First, 26.4 µL of HS-LBA was mixed with 873.6 µL of deionized water, and then 100 μL of THPC-Au NPs (particle concentration: 2.64 µM) was added to the mixture and the solution was stirred for 4 h. The LBA-Au NPs (particle concentration: 0.264 µM) could then be directly used without further purification.
However, in experiments for the specific targeting and selection of lysozyme for LBA-Au NPs, Hex-LBA-Au NPs were prepared by replacing HS-LBA-Hex with HS-LBA. Hex-LBA-Au NPs were purified using a high-speed centrifuge at 100,000g for 10 min to remove free Hex-LBA in the supernatant. The precipitate was redispersed and stored in deionized water at 4 °C for further use.
Preparation of Au+-complex solution
Cetyltrimethylammonium bromide (CTAB, 0.182 g) was dissolved in 9 mL of deionized water before adding 1 mL of HAuCl4 (5 mM). Subsequently, 60 µL of ascorbic acid (100 mM) was added to the mixture. The color of the solution changing from dark orange to colorless limpidity indicated the Au+-complex solution ([Au+] = 470 μM) was complete and ready for further use.
The absorption spectrum of the nanomaterials and biomolecules were determined using an ultraviolet–visible (UV–vis) spectrophotometer (HP8453; Agilent Technologies, Santa Clara, CA, USA).
Test of Au seed-mediated enhancement
First, 997.5 µL of the Au+-complex solution was added to 1 mL of eppendorf, followed by 2.5 µL of THPC-Au NPs (particle concentration: 2.64 µM). Following the mixing of the Au+-complex solution and THPC-Au NPs, the seed growth reaction process was monitored using a UV–vis spectrophotometer at intervals of 10 s for 5 min.
Collection of fingermarks
Professional-grade forensic tape was used to collect fingermarks. Volunteers first washed their fingers with soap and water and then dried them in air before pressing their fingertips on the tape. Fingermarks were collected immediately following deposition and after a 24 delay for further tests.
Rapid visualization of latent fingermarks
First, 200 µL of LBA-Au NPs (particle concentration: 132 nM) were spread over the fingermarks on the substrate. After incubating for 5 min, the sample was rinsed two times with deionized water to remove free LBA-Au NPs. Next, 200–300 µL of the Au+-complex solution was spread over the fingermark region, followed by incubation for 2–3 min. Notably, the required visualization time of the latent fingermarks was based on the quantity of lysozyme from different volunteers. In our experimental tests, the time required to visualize latent fingermarks did not exceed 5 min.
Results and discussion
In conclusion, we report a simple, inexpensive, and fast method for the rapid visualization of latent fingermarks on the non-porous substrates using Au seed-mediated enhancement. LBA-Au NPs were used as Au seeds with a Au+-complex solution composed of HAuCl4, CTAB, and AA used as a growth agent. The Au+ ion of the Au+-CTAB complexes could be only reduced from Au+ to Au0 in the presence of the Au seeds. Importantly, the procedure requires only two main steps for latent fingermark visualization and can be completed in less than 10 min. In the first step, latent fingermarks are incubated with LBA-Au NPs for 5 min. In the second step, the latent fingermarks are treated with the Au+-CTAB complex solution for 3 min. In fact, latent fingermarks can be visualized for observation in less than 3 min after treatment with Au+-complex solution and without background interference. The proposed approach offers faster detection and visualization of latent fingermarks than existing methods. Moreover, the results are observable directly with the naked eye, without the use of expensive or sophisticated instruments. The proposed method is expected to increase detection efficiency for latent fingermarks, thus reducing time requirements and costs for forensic investigations and medical diagnostics.
Study design: FYC. Data collection: CHS, CCY, FYC. Drafting manuscript: FYC. All the authors reviewed and approved the final manuscript. All authors read and approved the final manuscript.
This work was supported by Grant MOST 103-2113-M-006-005, 103-2633-B-182A-001, 103-2320-B-182A-004, and 103-2320-B-182A-004 -MY3 from the Taiwan Ministry of Science and Technology and CMRPG8E1461 from Kaohsiung Chang Gung Memorial Hospital, Taiwan.
The authors declare that they have no competing interests.
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