A novel approach to fingerprint visualization on paper using nanotechnology: reversing the appearance by tailoring the gold nanoparticles' capping ligands

Abstract

Gold nanoparticles, AuNPs, capped with mercaptocarboxylic acids followed by silver precipitation develop latent fingermarks on paper as high quality “negative” impressions. This effect stems from hydrogen bonding between the carboxylic group and the paper cellulose and may improve the yield of latent fingermarks since the results are less dependent on sweat composition.

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Recovering latent fingermarks from paper is a common task in police forensic work, but quite often the developed marks are very faint or their quality does not allow identification. The common approach for visualization involves the use of chemical reagents which form either a colour or luminescence with some of the chemical components of sweat thus rendering the latent marks visible.^1–3 A considerable portion of the latent marks still escapes detection since the amount of sweat in the latent marks may be extremely small and contain only very little amino acids, the common targeting compounds for chemical enhancement on paper.^4–6 “Reversed” or “negative” fingerprint development describes a process in which latent fingermarks are enhanced in a reversed way, namely, the reaction takes place outside the ridges and between them, but not on them. In this process, the fingermark material serves as a mask and does not participate in the chemical reaction. Perhaps the best known example of such a process is the VMD (Vacuum Metal Deposition) technique, for development of fingermarks on polymeric materials, which is considered the most sensitive fingerprinting technique for e.g. plastic bags.^7,8

Our group had shown recently that negative fingermarks can be obtained also on paper surfaces. It appears upon treatment with gold nanoparticles, AuNPs, which are capped with the bi-functional ligand S-11-[4,5-dichloro-6-oxopyridazine-1(6H)-yl]-11-oxoundecyl ethanethioate 1 (Fig. 1), followed by precipitation of silver on the gold-coated areas.⁹ While this strategy has the potential to increase the yield of latent fingermarks by law-enforcement agencies, the preparation of 1 requires several synthetic stages and a tedious work up.

Fig. 1 S-11-[4,5-Dichloro-6-oxopyridazine-1(6H)-yl]-11-oxoundecyl ethanethioate (1).

Another difficulty stems from the working medium which is a mixture of polar organic solvents (CH₃CN–DMSO), an undesirable system for document examinations, since it dissolves inks.

Based on the initial results,⁹ we anticipated that other ligands possessing: (a) “active head” which can be involved in hydrogen bonding with the paper cellulose and (b) “active tail” which can bind to gold nanoparticles, should also induce “reversed” fingermarks. We have therefore searched for simpler ligands, which, while fulfilling the two requirements, can also operate in aqueous solutions.

The immediate candidates were mercaptocarboxylic acids, many of which are commercially available. We assumed that their thiol groups can bind to AuNPs, and the carboxylic groups can form efficient hydrogen bonds with the paper cellulose. In fact, several AuNPs which are capped with mercaptocarboxylic acids have been described in the literature and fully characterized.^10–12 AuNPs capped with the following list of mercaptocarboxylic acids have been prepared and tested (Fig. 2).

Fig. 2 Mercaptoacids and salts used in this study.

Other similar bi-functional ligands that were tested were the sodium salts of the two mercaptoalkyl sulfonic acids 6 and 7.

AuNPs protected by the bi-functional ligands shown in Fig. 2 (except 5) were prepared in aqueous medium according to Kornberg et al.¹³ Ligand 5 is insoluble in water; therefore, AuNPs capped with this ligand were prepared in a slightly different manner, according to Sugimoto et al.,¹⁴ and re-dissolved in THF.

Sebum rich (sebaceous) fingermarks were obtained from several volunteers by rubbing their fingers against their forehead and stamping them onto A4 paper strips. The paper strips bearing the sebaceous fingermarks were dipped in an aqueous solution of AuNPs protected by ligands 1–9 (except 5) for about 12 min. THF solutions of AuNPs stabilized by ligand 5 were further diluted with water to obtain 5% v/v of THF in water. A silver physical developer, Ag-PD, was used in its regular mode as a secondary treatment to visualize the fingerprints.¹⁵ Immersion in Ag-PD typically takes 5–60 s and may take up to 1 h when AuNPs capped with ligand 5 are applied. It is important to rinse the paper with water after the malic acid prewash and immediately after the Ag-PD treatment.

Apart from ligands 4 and 5, AuNPs capped with all other new ligands (Fig. 2) induced the development of negative fingermark impressions. The quality of the marks varied from average (the sulfonic acid salts) to excellent, depending on the ligands and the age of the marks.

The best results were obtained using AuNPs capped with ligand 3, 3-mercaptopropionic acid. It developed fresh as well as 14 months old fingermarks as negative impressions with very good contrast and resolution (Fig. 3a and b).

Fig. 3 Fingermarks developed by AuNPs capped with mercaptocarboxylic acids followed by Ag-PD. (a) 3-Mercaptopropionic acid (fresh print). (b) 3-Mercaptopropionic acid (14 months old print). (c) 4-Mercaptobenzoic acid (fresh print). (d) 11-Mercaptoundecanoic acid (fresh print).

Latent fingermarks failed to develop when AuNPs capped with ligand 4, 6-mercaptohexanoic acid, were applied. Instead, the paper and the sebaceous ridges were completely stained. This is explained by a simultaneous adherence of the AuNPs to the paper and to the lipid ridges. Subsequently, a secondary treatment with Ag-PD led to dark staining on the entire paper including the ridges. In the case of ligand 5, 11-mercaptoundecanoic acid, with a much longer carbon chain, the AuNPs bind preferentially to the sebaceous ridges and thus develop the common “positive” fingermarks (Fig. 3d).

Aslam et al. showed that AuNPs stabilized with dodecanethiol in organic medium are adsorbed onto activated hydrophobic surfaces by hydrophobic interactions.¹⁶ We found that fingermark deposits can serve as the hydrophobic surface for such interactions and AuNPs capped with different n-alkanethiols adhere preferentially to the ridges of sebum-rich latent fingermarks. A clear relationship between the chain-length of the stabilizing thiols and the quality of the fingerprints was found: the longer the chains the clearer are the (positive) fingermarks.¹⁷ Here we show that when the capping ligands also possess a group which can form hydrogen bonds with the paper cellulose, such as carboxyl, the above behavior can be reversed, depending on the length of the ligand carbon chain.

With short-chain ligands, the association with the cellulose is apparently dominant, resulting in coating of the entire paper with gold and none or very little coating on the sebaceous areas.

This in turn leads to the deposition of silver around the ridges and between them but not on them, the outcome of which is a “negative” or “reversed” fingermark impression (after long immersion periods in the Ag-PD solution, the ridges also turn black, due to the ordinary but slower Ag-PD process in which metallic silver is deposited on the sebaceous material).¹⁵ From the experimental results it appears that the difference in performance between the various mercaptoacids stems from the difference between the two affinities. As a rule, in the mercaptoalkyl carboxylic acids series, the longer are the carbon chains of the acids, the higher is the affinity of the AuNPs to the sebaceous ridges (2-mercaptoacetic acid and 3-mercaptopropionic acid produced similar high quality prints).

In conclusion, we have shown that:

(1) “Negative” fingermarks can be consistently developed on paper items.

(2) The process requires AuNPs capped with ligands that can form hydrogen bonds with cellulose such as mercaptocarboxylic acids.

(3) The appearance of the marks, “positive” or “negative”, can be tuned by the length of the ligands' alkyl chains. Shorter chains lead to “negative” appearance and vice versa.

(4) The “reversed” strategy may improve the yield of latent fingermarks from paper items since the process is less dependent on the sweat composition.

This project was partially supported by the US Technical Support Working Group (TSWG). The authors wish to thank Dr Michal Elad-Levin, Mr Amir Magira and Mr Tomer Arbeli of the Israel Police Fingerprint Laboratory for their assistance throughout this project.

Notes and references

1. C. Champod, C. Lennard, P. Margot and M. Stoilovic, Fingerprints and Other Ridge Skin Impressions, CRC Press, LLC, Boca Raton, FL 33431, 2004.
2. R. R. Hark, D. B. Hauze, O. Petrovskaia and M. M. Joullié, Can. J. Chem., 2001, 79, 1632.
3. R. S. Ramotowski, in Lee & Gaensslen's Advances in Fingerprint Technology, ed. R. S. Ramotowski, CRC Press, LLC, Boca Raton, FL 33487-2742, 3rd edn, 2013.
4. S. Wiesner, E. Springer, Y. Sasson and J. Almog, J. Forensic Sci., 2001, 46, 1082.
5. J. Almog, H. Sheratzki, M. Elad-Levin, A. E. Sagiv, G. D. Singh and O. P. Jasuja, J. Forensic Sci., 2011, 56, S162.
6. M. Wood, P. Maynard, X. Spindler, C. Lennard and C. Roux, Angew. Chem., Int. Ed., 2012, 51, 12272.
7. X. Dai, M. Stoilovic, C. Lennard and N. Speers, Forensic Sci. Int., 2007, 168, 219.
8. N. Jones, M. Stoilovic, C. Lennard and C. Roux, Forensic Sci. Int., 2001, 123, 5.
9. N. Jaber, A. Lesniewski, H. Gabizon, S. Shenawi, D. Mandler and J. Almog, Angew. Chem., Int. Ed., 2012, 51, 12224.
10. T. Selvam, C.-M. Chiang and K.-H. Chi, J. Nanopart. Res., 2011, 13, 3275.
11. E. C. Cho, S.-W. Choi, P. H. C. Camargo and Y. Xia, Langmuir, 2010, 26, 1005.
12. G. Wang, R. Guo, G. Kalyuzhny, J.-P. Choi and R. W. Murray, J. Phys. Chem. B, 2006, 110, 20282.
13. P. D. Jadzinsky, G. Calero, C. J. Ackerson, D. A. Bushnell and R. D. Kornberg, Science, 2007, 318, 430.
14. J. Matsui, K. Akamatsu, S. Nishiguchi, D. Miyoshi, H. Nawafune, K. Tamaki and N. Sugimoto, Anal. Chem., 2004, 76, 1310.
15. A. A. Cantu and J. L. Johnson, Advances in Fingerprint Technology, ed. H. C. Lee and R. E. Gaensslen, CRC press, Boca Raton, FL, 2nd edn, 2001, pp. 242–247.
16. M. Aslam, I. S. Mulla and K. Vijayamohanan, Langmuir, 2001, 17, 7487.
17. M. Sametband, I. Shweky, U. Banin, D. Mandler and J. Almog, Chem. Commun., 2007, 1142.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc41610k

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