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Paper is one of the most frequently tested surfaces for the detection of latent fingermarks in criminal and terroristrelated investigations. Despite the plethora of modern physical and chemical fingerprint-detection techniques, a considerable portion of the latent fingermarks still escape detection. A plausible explanation is the remarkable difference in sweat composition between individual persons. A potential remedy to this problem could be achieved by reversing the roles, that is, developing a fingerprint-detection technique in which paper serves as the substrate for the interaction with the reagent, while the latent fingermarks will serve as a mask. In this work, “negative” fingermarks have been developed on paper, even after soaking in water, by the application of a new bifunctional reagent attached to gold nanoparticles (AuNPs), and then a silver physical developer (Ag-PD). The bifunctional reagent is composed of an active head, that is, a polar group with high affinity to cellulose, attached by a long chain to an active tail containing a sulfur group, which can stabilize AuNPs. Through the active head, the AuNPs, which are stabilized by the active tail, adhere preferentially to the paper cellulose rather than to the fingerprint material, to which they conventionally bind. Consequently, Ag-PD, which normally develops sebaceous fingermarks by precipitating dark silver on the sebaceous material, precipitate preferentially on the gold-coated areas giving rise to the appearance of uncoloured ridge detail on a dark background. In this competing process, the paper itself serves as the substrate, whereas the fingermarks serve as a mask. This process may increase the overall yield of developed fingermarks as it bypasses the issue of the remarkable differences in sweat composition between individual persons. Functionalized nanoparticles have drawn great interest during the past decade not only in potential applications as biomedical, electronic, catalytic, and optical materials, but also in forensic science, as visualizing reagents for latent fingermarks. Accumulative results on a large number of paper items such as used checks, all of which are supposed to bear latent fingemarks, have shown that over 50% escape detection of identifiable marks. Such observations have led to intensive research into more-sensitive fingerprint detection techniques. Paper that has been wetted is a particular challenge since the amino acids, which are the main substrate for chemical enhancement of latent fingermarks, are dissolved and removed by the water. A silver physical developer (Ag-PD), comprising an aqueous solution of silver nanoparticles (AgNPs) stabilized by cationic surfactants, is required to achieve satisfactory development of such fingermarks. Silver slowly deposits on the water-insoluble components of the sweat, forming dark grey to black impressions (Figure 1). Although the technique is quite sensitive, it suffers from several inconveniences, including complexity, lack of reproducibility, solution instability, and often poor contrast. 6] As a result, many forensic laboratories refrain from using this technique on a routine basis. Latent fingermark enhancement by gold nanoparticles stabilized by citrate ions in aqueous medium, and then a modified Ag-PD, is currently used in a process known as colloidal gold or multimetal deposition (MMD). AuNPs adhere to the fingermark residue and catalyze the precipitation of metallic silver from the Ag-PD solution. The gold adherence to the fingermark material is explained by an ionic interaction between the negatively charged gold colloids and the positively charged components of the fingermark residue at low pH. In a modification known as SMD (singlemetal deposition), the enhancement of gold colloids by precipitation of silver, was replaced by gold-based growth of the nanoparticles. Several other fingerprint techniques that are also based on nanochemical processes, have been suggested recently; all of these suggestions involve the adherence of nanoparticles to the fingerprint material. 3, 8] Previously we showed that good quality fingermarks were developed by treatment with an organic solution of hydrophobic AuNPs stabilized by long chain thiols, and then with Figure 1. Sebaceous fingermark developed by Ag-PD. |
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