TEOS and OTS were purchased from Alfa Aesar (Shanghai, China). Ethanol, ammonia and n-hexane were from Beijing Chemical Reagents Company (Beijing, China). All the chemicals were of analytical grade and were used as received without further purification. Deionized water was used throughout the research work.

Monodisperse silicon dioxide spherical particles were obtained in situ by mixing TEOS with ethanol and deionized water. Ammonia was used as a catalyst during the hydrolysis and condensation processes. OTS was used as a surface modifier to obtain amphiphilic silica particles. More specifically, the following procedure was used to obtain amphiphilic silica nanoparticle. In the first step, ethanol, deionized water and ammonia (25%) were added into a three-necked flask in sequence, and stirred for about 30 min at room temperature. Different quantities of TEOS were then added (0.5, 1 and 2 g) to obtain particles with different diameters. The solution was stirred for 8 hours and then washed 5 times using high-speed centrifugation at 5500 r/min (TDL-5-A, Shanghai Anting Scientific Instrument Factory, Shanghai, China). Finally, the monodisperse hydrophilic silicon dioxide spherical particles were dried in a vacuum oven (202-l, Shanghai Yiheng Instruments Co., Ltd., Shanghai, China) for 24 hours. Particles with three average diameters of 250, 550 and 700 nm were obtained. In the second step, the monodisperse hydrophilic silicon dioxide spherical particles were modified by reducing 1 g of silica powder in 100 mL n-hexane. The silica particles were dispersed 30 min using an ultrasonic oscillator. 0.01, 0.02, 0.1 and 0.5 g of OTS were added to obtain particles with the following SiO₂: OTS weight ratios: 100:1, 50:1, 10:1 and 2:1. The solution was stirred for 12 hours. The amphiphilic silica nanoparticles were washed twice using high-speed centrifugation at 5500 r/min and dried in a vacuum oven for 24 hours.

The morphology of the amphiphilic silica nanoparticles was examined by field-emission gun scanning electron microscope (FEG-SEM) of a Hitachi S-4800 type (Tokyo, Japan) equipped with a high-resolution secondary electron detector (in lens detector), operated at 10.0 kV and a point-to-point resolution of 2 nm. The optical properties of the silica nanoparticles were characterized using Fourier transform infra-red spectrometer (Perkin-Elmer Corporation, Waltham, USA). Spectra were recorded using the 514.5 nm line of a helium-neon laser at a power of 150 W and spectral resolution of 1 cm^-1. The X-ray photo electron spectrum (XPS) of the amphiphilic silica nanoparticles was obtained using an ESCALAB250Xi (Thermo Fisher Scientific Ltd., Waltham, USA) hemispherical electron analyzer, and an aluminum anode as the X-ray source. The X-ray source was operated at 12 keV and 15 mA. Binding energies set at 103.4 eV were corrected using the reference Si2p line from silica, X-ray powder diffraction (XRD) analysis was performed on a Rigaku D/max-2500 diffractometer (Rigaku Corporation, Tokyo, Japan) equipped with Cu Kα radiation (λ =1.54178Å ) operating in continuous scan mode at 2° min^-1. The measurements were collected at room temperature with a scan range between 3° and 60° .

The performance of amphiphilic silica nanoparticle powder for the detection of latent finger impressions was measured using a series of 300 latent fingerprints. The fingerprints were obtained from 10 donors, who were asked to wash their hands thoroughly with soap. Each one of the 10 fingers from each donor was gently rubbed across the donor’ s forehead and deposited on clean microscopy glass slides. This process was repeated 3 times for each donor, resulting in 3 sets of 100 latent prints. The sample sets were kept at room condition for 1, 3 and 7 days before processed with the silica nanoparticle powder.

Detection of the latent fingerprint was performed using the different particles obtained as described above (i.e., particles with different diameters and different SiO₂: OTS weight ratios). Brushing was done using squirrel hair fingerprint brushes. Fingerprint images were captured using a DCS4 fingerprint enhancement system (Foster & Freeman Ltd., Evesham, UK).

During the preparation of the monodisperse hydrophilic silica nanoparticles, TEOS was hydrolyzed to form the corresponding hydroxylated products and alcohol; meanwhile, the condensation reaction between silicic acid itself and TEOS occurred. The reaction equations show as below:

Si(OR)_n+xH₂O→ Si(OH)_x(OR)_n-x + xROH

-Si-OR + HO-Si-→ Si-O-Si + ROH

-Si-OH + HO-Si-→ Si-O-Si + H₂O

During the modification step of the procedure, OTS reacted with the silanols from the surface of the silicon spheres, resulting in the loss of Cl^- and H⁺; eventually Si-R and Si-OH groups coexisting at the surface of the silica particles. If the concentration of the OTS was high enough, the hydroxyl groups on the surface of the nanoparticles were almost all replaced, causing lipophilic nano-SiO₂ composite particles with Si-R groups on their surfaces. By adjusting the weight ratios of OTS and TEOS, the ratio of Si-R and Si-OH groups on the silica nanoparticle could be changed, therefore, different levels of modified amphiphilic silica particles were offered.

3.2.1 FEG-SEM Images

The FEG-SEM images of the hydrophilic silica material before modification showed that the particles were highly soluble in water, but not in hexane (Fig.1). This implied that hydrophilic silica particles would not dissolve in the amino acid component of latent fingerprint residue. This was the main reason why hydrophilic silica particles were not appropriate for detecting latent fingerprints. On the other hand, silicon spheres, modified by OTS, showed excellent solubility in hexane. The solubility appeared to increase with the weight ratio (WR) of SiO₂ to OTS (Fig.2). According to the results, the bigger the weight ratio of SiO₂ to OTS, the better the nanoparticles adhere to each other. The weight ratio of SiO₂ to OTS became the key factor to affect the ability of amphiphilic silica nanoparticles to detect latent fingerprints.

Fig.1

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	Fig.1 SEM images of the hydrophilic silica particles in water bath (a, b) and in hexane bath (c, d)

Fig.2

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	Fig.2 SEM images of the amphiphilic silica particles in hexane bath with the weight ratio of SiO2: OTS at 2:1 (a), 10:1 (b), 50:1 (c) and 100:1 (d), respectively

3.2.2 FTIR Spectra for Amphiphilic Silica Nanoparticle

Fourier transform infrared spectra provided important corresponding data for the characterization of the structure of the nanoparticles in the amphiphilic silica material by offering information on the microstructure of the material. The spectra of silica sphere before modification could be seen in Fig.3a. In Fig.3a, three peaks at 797, 1100 and 1400 cm^-1 were observed. The peak at 1100 cm^-1 was the antisymmetric stretching vibration, belonging to Si-O-Si, and the peak at 1400 cm^-1 was the stretching vibration of C-O. These two peaks showed that some Si-C groups were present on the surface of the silica sphere before modification. The peaks at 935 and 3414 cm^-1 reflected the bending vibration of Si-OH and the stretching vibration of -O-H. On Fig.3b~3d, the peaks near 2921 and 2852 cm^-1 reflected the antisymmetric stretching vibration arising from -CH₂- alkyl chain, which showed that alkyl groups had successfully replaced some of the hydroxyl groups on the surface of silica spheres. The peak at 1469 cm^-1 reflecting the asymmetric bending vibration of -C-H-, also explained above view. The peak near 1632 cm^-1 might be caused by the absorbed water during the particlesʼ preservation process. Fig.3a-3d showed that the intensity of the characteristic absorption peaks gradually increased with the rise of the weight ratios. It also meant the function of adherence could be strengthened by the increase of the weight ratios.

Fig.3

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	Fig.3 FTIR spectra of unmodified silica sphere (a), amphiphilic silica particle modified by the weight ratio of SiO2׃ OTS at 100׃1 (b), 50׃1 (c) and 10׃1 (d)

3.2.3 X-ray Photoelectron Spectroscopy

XPS analysis was conducted in order to obtain information about the surface and the feature of the chemical bonds. Fig.4 showed the XPS binding energy (eV) values of C1s, O1s and Si2p were acquired for the silica nanoparticles. The binding energy (B.E.) value of C1s at 284.7 eV was assigned to carbon bonded atoms in polyaromatic structures in graphite (B.E.¼ 284.6 eV), or carbon presented in alcohol^{[32, 33, 34, 35]}. As a consequence, the binding energy value of O1s at 532.8 eV was assigned to carbonyl oxygen atoms in esters, and anhydrides and oxygen atoms in hydroxyl groups. This implied that the modified silica material was ideal for further efficient chemical modifications.

Fig.4

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	Fig.4 XPS images of unmodified silica sphere (a), amphiphilic silica particle modified by the weight ratio of SiO2: OTS at 50:1 (b), 10:1 (c)

3.2.4 X-ray Powder Diffraction

To show that the silica material had a disordered structure, the XRD patterns of the unmodified silica sphere and amphiphilic silica nanoparticle were obtained and compared (Fig.5). It showed that both the unmodified material and amphiphilic silica presented an amorphous structure, which could be inferred from the absence of peak reflection in the XRD diffractogram. Based on these results, it was safe to say that neither of the particle types had periodicity, but both were disordered.

Fig.5

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	Fig.5 XRD images of unmodified silica sphere (a), amphiphilic silica particle modified by the weight ratio of SiO2: OTS at 10:1 (b)

The dispersion performance of amphiphilic nanoparticle in water and organic solvent was studied (Fig.6). The modified silica material was found well-dispersed in hexane environment, indicating that amphiphilic silica particles were highly lipophilic. To facilitate the observation, a small amount of methyl orange was added to color the water. Silica nanoparticles of 1% mass fraction was added to the water and hexane mixed system (water and hexane volume ratio is 1:1). It showed that the modified silica nanoparticles were well dispersed in the hexane phase.

Fig.6

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	Fig.6 Modified silica particle in hexane environment (a), reference solutions of 1:1 water and hexane (b), and modified silica particle in mixed system of water and hexane (c)

3.4.1 Effect of Weight Ratio of Unmodified Silica Sphere to OTS

The images acquired using the SEM (Fig.2) showed that as the weight ratio of SiO₂: OTS increased, adhesive ability between particles improved. In order to find the most effective amphiphilic silica particle for detecting latent fingerprint, the effectiveness of 4 different weight ratios (100׃1, 50׃1, 10׃1, 2׃1) was compared. All types of modified particles were used to detect fresh fingerprints, which were left on glass surfaces (Fig.7). Even though all of modified nano-silica particlesʼ powder appeared to be able to detect latent fingerprints, the best weight ratio of SiO₂: OTS was 10:1.

Fig.7

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	Fig.7 Images of latent fingerprints developed by amphiphilic silica particle which was modified by the SiO2: OTS weight ratio of 100:1 (a), 50:1 (b), 10:1 (c) and 2:1 (d)

3.4.2 Effect of Particle Diameter

The diameter of the amphiphilic silica particles depended on the hydrophilic silica material used. SiO₂ nano-particles prepared by the Stober method usually had uniform spherical surface with a large number of bonded hydroxyl groups at different states (e.g., isolated hydroxyl, hydrogen bonded hydroxyl and twins hydroxyl). Many factors might influence the preparation of the monodisperse silica spherical particles, such as the type of catalyst, the concentration of silicon source, the type of solvent, hydrolysis temperature, and reaction time. In the case of using ammonia as a catalyst, its concentration was the main factor influencing the particle size, followed by the silicon source density.

Different sizes of amphiphilic silica particles (250, 550 and 700 nm) were synthesized. Results showed that both the size of the nanoparticles and the weight ratio of SiO₂: OTS could affect the fingerprint detection. The best diameter for the amphiphilic silica particles (DASP) was 700 nm and best weight ratio of SiO₂: OTS was 10:1 (Table 1).

Table 1

Table 1

Table 1 Summary of the results obtained when different sets of latent fingerprints on glass slices developed with different weight ratio of SiO2: OTS and different diameter of modified silica particles WR of SiO2: OTS 100:1 50:1 10:1 2:1 DASP diameter (nm) 250 550 700 250 550 700 250 550 700 250 550 700 1day fingerprints ++ ++ +++ ++ ++ +++ ++ ++ +++ ++ ++ +++ 3days fingerprints ++ ++ ++ ++ + ++ +++ ++ ++ + ++ ++ 7days fingerprints ++ ++ +++ ++ ++ +++ ++ ++ +++ ++ + ++ +++: clear fingerprint image with sufficient quality to show minutiae; ++: fingerprint image showinng most of ridge; +: weak fingerprint image showing sporadic ridge.

Table 1 Summary of the results obtained when different sets of latent fingerprints on glass slices developed with different weight ratio of SiO2: OTS and different diameter of modified silica particles

3.4.3 Results of Fingerprints Development

The modified amphiphilic silica particles with a diameter of 700 nm and a weight ratio of SiO₂: OTS at 10:1 were used to develop latent fingerprints on nonporous objects such as glass, ceramic and plastic. With a subtle size, the high lipophilic particle could absorb onto the fingerprintsʼ oil residual much well. Compared with the traditional powders as golden, silver and magnetic powder, the amphiphilic silica nanoparticle could generate much more recognizable fingerprint image with continuous ridges and visible pores (Fig.8).

Fig.8

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	Fig.8 Images of latent fingerprints on glass developed by nanoparticle (a), golden powder (b), silver powder (c) and magnetic powder (d)