Fingermark detection on non-porous and semi-porous surfaces using NaYF4:Er,Yb up-converter particles Rongliang Ma a, Elicia Bullock a, Philip Maynard a, Brian Reedy a, Ronald Shimmon a, Chris Lennard b, Claude Roux a,*, Andrew McDonagh a a Centre for Forensic Science, University of Technology Sydney, Sydney, NSW 2007, Australia b National Centre for Forensic Studies, University of Canberra, Canberra, ACT 2601, Australia 1. Introduction Fingermark detection techniques based on luminescence are well-established and can be very sensitive on a variety of surfaces [1���3]. These techniques can provide greater contrast between the developed fingermark and the background compared to non- luminescent methods [1,3]. Using a suitable light source for excitation and a barrier filter on the imaging system, developed marks may be largely free of background interference provided that the substrate itself is not luminescent under the same conditions. However, there are a number of surfaces where conventional luminescent techniques provide poor contrast because of background luminescence. This is due to the fact that many consumer products exhibit luminescence under the condi- tions used for traditional luminescent fingermark detection techniques [1,4]. These products commonly have brightly coloured surfaces containing dyes that are designed to emit broadband luminescence. Printed surfaces with complex, non-luminescent patterns may also be difficult to image because the printed pattern may interfere with the fingermark ridges in the image banknotes are one example where this problem may arise. To date, fingermark development studies have focussed on luminescent materials that emit light of a longer wavelength than that of the excitation source. The difference in energy between the excitation and emission energy maxima is known as the Stokes shift, and for a single photon absorption process the Stokes shift is to lower energy (i.e. emission occurs at a longer wavelength than for the excitation). Anti-Stokes luminescence applications in the forensic science or security domains have been previously reported, for example, for the detection of anthrax [5] or as security ink in high-valued security documents [6]. In the current work, we explore the application of anti-Stokes luminescent materials (or so-called ������up-converters������) for finger- mark detection. In these materials, two photons are absorbed followed by emission of a single photon of higher energy (i.e. shorter wavelength ��� Fig. 1). Importantly, anti-Stokes lumines- cence is quite rare in most materials [7] and so this provides an excellent opportunity to develop and image fingermarks that are virtually free of background interference. A number of rare-earth complexes have been discovered that can absorb two photons and then transfer the energy to a dopant atom, which subsequently emits a single photon [8,9]. The material chosen for investigation in the current work is sodium yttrium tetrafluoride doped with erbium and ytterbium (NaYF4:Er,Yb). This compound is an efficient Forensic Science International xxx (2010) xxx���xxx A R T I C L E I N F O Article history: Received 29 June 2010 Received in revised form 18 September 2010 Accepted 21 September 2010 Available online xxx Keywords: Latent fingermarks Anti-Stokes materials Luminescence Fingerprint powders Cyanoacrylate stains Sticky-side powder Up-converters A B S T R A C T This article describes the first use of an anti-Stokes material, or up-converter, for the development of latent fingermarks on a range of non-porous surfaces. Anti-Stokes materials can absorb long-wavelength light and emit light at a shorter wavelength. This property is unusual in both natural and artificial materials and so fingermark detection techniques based on anti-Stokes luminescence are potentially sensitive and selective. Latent fingermarks on luminescent and non-luminescent substrates, including Australian polymer banknotes (a well-known ���difficult��� surface), were developed with sodium yttrium tetrafluoride doped with erbium and ytterbium (NaYF4:Er,Yb) by dry powder, wet powder, and cyanoacrylate staining techniques. This study illustrates the potential of up-converter phosphors for the detection of latent fingermarks. �� 2010 Elsevier Ireland Ltd. All rights reserved. * Corresponding author at: Centre for Forensic Science, University of Technology Sydney, PO Box 123, Broadway, Sydney, NSW 2007, Australia. Tel.: +61 295141718 fax: +61 295141460. E-mail address: Claude.Roux@uts.edu.au (C. Roux). G Model FSI-6222 No. of Pages 5 Please cite this article in press as: R. Ma, et al., Fingermark detection on non-porous and semi-porous surfaces using NaYF4:Er,Yb up- converter particles, Forensic Sci. Int. (2010), doi:10.1016/j.forsciint.2010.09.020 Contents lists available at ScienceDirect Forensic Science International journal homepage: www.elsevier.com/locate/forsciint 0379-0738/$ ��� see front matter �� 2010 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.forsciint.2010.09.020

anti-Stokes fluorescent material [8,9] that absorbs light in the near infrared (NIR) region of the spectrum and emits green visible luminescence (see Supplementary Data). We have assessed the use of the commercially available anti- Stokes material, NaYF4:Er,Yb, without modification, for the development of latent fingermarks on a range of non-porous surfaces and two semi-porous surfaces (Australian polymer banknote and magazine paper). 2. Materials and methods 2.1. General NaYF4:Er,Yb powder (Artemis Limited, UK) and cetyltrimethylammonium bromide (CTAB, BDH Laboratory Supplies, UK) were used as received. These powders were characterised by scanning electron microscopy/X-ray analysis and X- ray diffraction. Scanning electron microscope images were obtained using a LEO Supra 55 VP microscope (Zeiss) equipped with an in-lens secondary-electron detector. Energy dispersive X-ray spectra were obtained using an SEI and Phillips XL 30 series environmental scanning electron microscope with an accelerating voltage of 20 kV. X-ray diffraction experiments were performed using a Siemens D5000 X- ray diffractometer with a graphite post monochromator with the following parameters: wavelength 1.5406 A �� (Cu Ka), tube power 1.6 kW (40 kV at 40 mA), step size = 0.028, time per step = 3 s, divergent slit = 18, receiving slit = 0.02 mm, scan angle range = 3���808. Six donors supplied latent fingermarks for this work. Fingermarks were deposited after the donors wiped their fingers over their forehead or facial area just prior to fingermark deposition. A collection of fingermarks on glass from two donors was stored for up to 19 months for experiments on aged fingermarks. Surfaces investigated in this work include: glass (microscope slides), plastics (polyethylene bags, polystyrene/acrylic telephone handsets, polyester playing cards, polystyrene Petri dishes, and soft drink labels), aluminium foil, ceramic plates, beer cans, glossy paper and electrical tape (PVC). Australian polymer banknotes were also tested as they are known to be a difficult semi-porous surface for fingermark development [10]. The coating of these banknotes is a modified polyurethane lacquer over patterned offset and intaglio printing and each of the denominations exhibited broadband luminescence. Developed fingermarks were illuminated either with white light (for conventional reflection mode imaging) or by a laser diode system (either a Roithner RLDH-980-200-3 or a Techlasers Enforcer) operating at a wavelength of 980 nm (for upconverting mode imaging). The laser output power (measured using a laser power meter Laser Check, Edmund Optics Inc) was found to vary from 100 to 30 mW (Roithner laser) or 700 to 100 mW (Techlasers Enforcer) as the charge of the batteries depleted. Thus, the use of fully charged batteries is recommended. The area covered by the beam of these laser diode systems is typically of the order of 15���50 mm2. For this reason, during our experiments, the area of interest was manually scanned while a long shutter speed (2���20 s) was generally used. Images were recorded using either a Rofin Poliview (Rofin Australia Pty. Ltd.) or with a video spectral comparator (VSC 2000/HR Foster and Freeman, UK). When imaging in upconverting mode, an infrared blocking filter was used to exclude light of wavelength longer than 700 nm from the detector. 2.2. Fingermark development with dry powder NaYF4:Er,Yb powders were applied to fingermarks on the various surfaces using squirrel brushes (Optitech Company, Australia). For comparison purposes, finger- marks were also developed using aluminium powder and black powder (Lightning Powder Company, USA) using the same technique. A number of comparisons using fingermark halves are included in the Supplementary Data. 2.3. Fingermark development using a wet powder method To further investigate the utility of NaYF4:Er,Yb, the material was assessed for use in a wet powdering procedure. The powder was dispersed in aqueous media and applied to fingermarks on a range of nonporous surfaces. Fingermarks were developed using one of the following three methods. 1. Powder suspension using surfactants: NaYF4:Er,Yb powder (0.5%, w/v) was added to a solution of cetyltrimethylammonium bromide (0.1%, w/v) in Milli Q water and stirred for 10 min using a magnetic stirrer. The suspension was poured in Petri dishes and fingermark specimens were immersed in the suspension for around 3 min and then rinsed with deionised water. 2. Homogenised suspension: NaYF4:Er,Yb powder (0.5%, w/v) was added to Milli Q water in a glass beaker and agitated using an IKA Ultra-Turrax T25 digital homogenisation instrument at 6000 rpm for about 3 min. Fingermark specimens were then developed as described above. 3. Sticky-side wet powder: A slurry of NaYF4:Er,Yb powder in Photoflo 200 (Eastman Kodak Company)/deionised water (50/50 mix) was prepared and then painted over the latent fingermark area, allowed to develop for 30 s and then rinsed off using deionised water. Traditional silver sticky-side powder (Lightning Powder Company Inc, USA) was also used for comparison. 2.4. Cyanoacrylate staining Cyanoacrylate fuming and staining was applied to latent fingermarks on the glass microscope slides, ceramic plates, playing cards and plastic containers. Fingermarks developed with cyanoacrylate vapour were treated with NaYF4:Er,Yb either as a dry powder or as a suspension in hexane (subsequently rinsed, in this latter case, with hexane to remove background staining). Rhodamine 6G (Sigma��� Aldrich) was used to treat one cyanoacrylate-developed latent mark as a comparison. 3. Results and discussion 3.1. Characterisation of NaYF4:Er,Yb powder Fig. 2 shows a representative image and reveals that the particles have sizes ranging from 0.2 to 2 mm. Energy dispersive X-ray spectroscopy confirmed the presence of oxygen, fluorine, sodium, ytterbium, yttrium, and erbium in the material. Two crystal phases of NaYF4:Er,Yb have been reported, the a- cubic and b-hexagonal phases [8]. Analysis by X-ray diffraction (see Supplementary Data) showed that the particles mostly consisted of the b-hexagonal phase, although some a-cubic phase was also evident. Importantly, the luminescence intensity of the b- hexagonal phase is approximately ten times greater than that of the a-cubic phase [9]. 3.2. Fingermarks developed with NaYF4:Er,Yb powder The NaYF4:Er,Yb powder showed a good affinity for fingermark ridges on glass, plastic and aluminium surfaces with the exception of fingermarks on polyester playing cards, where extensive background staining was evident and no latent fingermarks could Fig. 1. Schematic representation of the anti-Stokes luminescence emission from a NaYF4:Er,Yb particle (each arrow represents one photon). Fig. 2. Scanning electron micrograph of NaYF4:Er,Yb powder. R. Ma et al. / Forensic Science International xxx (2010) xxx���xxx 2 G Model FSI-6222 No. of Pages 5 Please cite this article in press as: R. Ma, et al., Fingermark detection on non-porous and semi-porous surfaces using NaYF4:Er,Yb up- converter particles, Forensic Sci. Int. (2010), doi:10.1016/j.forsciint.2010.09.020