Improved discrimination of pen inks on document by surface-enhanced Raman substrate fabricated by magnetron sputtering

Optik - International Journal for Light and Electron Optics 201 (2020) 163499

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Original research article

Improved discrimination of pen inks on document by surfaceenhanced Raman substrate fabricated by magnetron sputtering

T

Noppadon Nuntawonga,⁎, Saksorn Limwicheana, Mati Horprathuma, Viyapol Patthanasettakula, Apinya Ketkongb, Kheamrutai Thamaphatb, Prapapun Petchruangrongc, Sujinda Jankongc, Paweena Kasikijwiwatc, Pongpan Chindaudoma, Pitak Eiamchaia a

Opto-Electrochemical Sensing Research Team (OEC), National Electronics and Computer Technology Center, 112 Thailand Science Park, Pathum Thani, Thailand b Applied Science and Engineering for Social Solution Research Group, Department of Physics, Faculty of Science, King Mongkut’s University of Technology Thonburi, Bangkok, Thailand c Forensic Document Examination Section, Central Institute of Forensic Science, Bangkok, Thailand

ARTICLE INFO

ABSTRACT

Keywords: Raman SERS Silver Nanorod Ink Document examination

For questioned document examinations, conventional Raman spectroscopy often encounter with weak signal intensity and/or fluorescence interfering. The surface enhanced Raman spectroscopic (SERS) technique can efficiently accommodate the issues, by greatly improving the Raman intensity by several orders of magnitude. However, colloidal SERS fabricated by chemical reduction method practically suffer from native contaminant and uncontrollable aggregation on document surface. In this work, we report an alternative surface-enhanced Raman scattering technique for analysis of pen inks on written document by utilizing film-based Ag nanorod SERS substrate. Highly improved discriminating powers (DP) were obtained from SERS substrates. The pen samples consisted of ninety-five blue and black ballpoint pens, of oil-, water-, and gel-based, indiscriminately acquired from local sources in Thailand. The Raman spectroscopic (RS) and SERS analyses were performed with the confocal Raman spectrometer at 785 nm excitation wavelength. The spectral data were directly collected from both the written pen inks and the traces of the inks with the SERS substrate. The results could categorized the pen ink samples into either the RS or SERS active/inactive results, and further calculated and analyzed as the discriminating powers (DP). The results indicated that the proposed SERS method was highly effective at distinguishing the ballpoint pens on the written documents with the obtained DP values at 0.79 and 0.92 for the blue and black pen samples, respectively.

1. Introduction For questioned document examinations, ballpoint pens have been routinely investigated by forensics laboratories since becoming popular more than 5 decades ago [1]. Although sharing the same ballpoint mechanism, the ballpoint pens have been classified into 3 types, corresponding to their different liquid media, i.e., ballpoint pen, rollerball pen, and gel pen for oil-based, water-based and gelbased components, respectively. Alterations with different pens but very similar color shades are sometimes not visible to the human

⁎

Corresponding author. E-mail address: [email protected] (N. Nuntawong).

https://doi.org/10.1016/j.ijleo.2019.163499 Received 30 May 2019; Received in revised form 23 September 2019; Accepted 28 September 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.

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eye and difficult to detect even through the use of an enhanced optical imaging that utilize multispectral of incident, reflected and emitted lights [2]. Combination of chromatographic methods, e.g., thin layer chromatography (TLC), high performance liquid chromatography (HPLC) and Gas chromatography–mass spectrometry (GC–MS) have been highly effective for distinguishing inks by their chemically differences. Nevertheless, such techniques require lengthy sample preparation and the measurement needed to be done in laboratory due to nature of complicated and bulky instruments. Today, Raman spectroscopy has especially become more attractive to forensic investigator due to technical innovation of several spectrometer components over the past few years. The modern generation of Raman spectrometer features miniature laser diode module and more sensitive thermoelectrically cooling system, which offer much greater portability, higher sensitivity, shorten time acquisition and lower price. For these reasons, characterization by the technique is expected to find more applications in homeland securities and forensics [3]. For document analysis, the Raman spectroscopy technique has been capable of pen inks discrimination from their molecular vibrations of distinct dyes and other components [4–10]. Establishment of modern Raman spectroscopy for document investigation laboratory continues growing in number as result of increasing in cases related to the falsification and manipulation of documents. Nevertheless, when dealing with conventional Raman spectroscopy, analyses often encounter with weak signal intensity and/or fluorescence interfering. In addition, the ink on document can be easily denatured with increasing laser power in order to obtain higher signal intensity. These have been the inherit problems for document investigation using the technique [11]. The surface enhanced Raman scattering (SERS) technique, which mainly based on electromagnetic and chemical enhancements of molecules when they are adsorbed in the proximity of noble metallic nanostructure, has drawn a great attention in research and development for trace molecular detections [12–14]. For document investigation, the SERS technique can effectively accommodate the issues by greatly improving the Raman intensity by several order of magnitude [1516], and raising the discrimination power as the results [17–19]. Although, few types of SERS substrate are available, to date, there have been only reports for colloidal nanoparticle SERS contributing to ink on document analyses. [11,15–17] Despite impressive demonstration, colloidal SERS fabricated by chemical reduction method practically suffer from native contaminant and uncontrollable aggregation on document surface due to instability of nanomaterial on complicate solution system. In most cases, the substrate needed to be prepared shortly before measurement. These factors make it difficult to produce reliable and repeatable measurements. On the other hand, the invention of a convenient SERS fabrication technique based on high vacuum coating methods, i.e., oblique angle deposition (OAD) and glancing-angle deposition (GLAD), have gained much attention due to contaminant-free process, cost-effectiveness, repeatability, and yields of high-performance SERS substrates on a large scale [20,21]. It is noteworthy to mention that GLAD-based SERS substrates developed by our group are now available for market at a pilot scale [22,23]. Although, the thin-film based substrates have been reported for its potential to be utilized for solving several problems in forensic sciences [21–25], there is still no study exploiting the use of such substrate for applications in document investigations. In this work, we report the systematic study of utilizing thin-film based SERS substrate for analysis of ballpoint pen on written document. The pen samples used in this study consists of blue and black ballpoint pens randomly collected from both local stationery stores and collected evidences from Central Institute of Forensic Sciences (CIFS), Thailand, in order to simulate the most commonly used ballpoint pen inks for legal transactions in Thailand. For Raman spectral acquisitions, an NIR laser with an excitation wavelength of 785 nm was chosen in this study because the particular laser line have become increasingly favorable. The reason was partly from the readily available bench-top and handheld spectrometers which utilized such laser excitation wavelength, and partly from superior performance with limited fluorescence interferences. [26] Results obtained from both conventional Raman spectroscopy and SERS were analyzed and discussed. 2. Materials and methods 2.1. SERS fabrications Large scale and reproducible SERS substrates were prepared by a laboratory-made dc magnetron sputtering system, with a glancing-angle deposition (GLAD) technique. The sputtering target was a 3-inch silver (99.99%, K.J. Lesker), and the substrates were p-type silicon (100) wafers mounted on the custom-designed substrate holder. During the SERS preparation, highly ordered nanostructures of silver nanorods were deposited with a DC power supply at 0.5 A and 340 V. The deposition rate was approximately 1.0 nm/s as calibrated by a spectroscopic ellipsometer (J.A. Woollam, HTC-190). Substrate rotation during deposition was introduced for the formation of vertically-aligned nanorods. Other deposition conditions were designed such that, the nanorod arrays were spaced out for high porosity. Verified by field-emission scanning electron microscopy (FE-SEM; Hitachi, S-5200), the verticallyaligned nanorod dimension was approximately 1.0 μm in length and 100 nm in width, as illustrated in Fig. 1(a)–(b). In our previous report [22,23], the similar vertically-aligned nanorod structure has been proved for highly sensitive SERS detectability of trace molecules. After the deposition, the fabricated samples were cleaved into small pieces of 5 × 5 mm2 to be mounted on top of glass slides. The mounted samples were handled with great care and stored in nitrogen-filled metalized packages prior to usage. 2.2. Ballpoint pen specimens To represent commonly used inks of ballpoint pens for legal transactions in Thailand, a sample set of ballpoint pens were indiscriminately acquired from local stationery stores and the CIFS’s evidence room. The sample set was collected initially without 2

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Fig. 1. (a) top view and (b) cross-sectional FE-SEM images of the fabricated SERS substrate.

consideration to brand names or types of pen inks. Subsequently, detailed information of the pen samples, i.e., brand names, ink colors, and ink types, was determined and summarized. From the total of ninety-five ballpoint pen samples, forty-nine and forty-six pens dispensed blue and black inks, respectively. To simulate a controlled environment, the authors collected pen ink specimens, each from handwritten lines on a single sheet of plain white office paper (Double A, A4 size, manufactured by Double A (1991) Public Company Limited, Thailand) on exactly the same day. Due to its popularity in local markets, this paper product most likely represented common usages with the ballpoint pens under normal circumstances. In addition, this type of paper exhibited low fluorescence background with intensities less than 5% of the full scale range of the spectrometer, which allowed great advantages in this study. Finally, all the pen ink samples were carefully cataloged and kept in a controlled storage environment, in the range of temperatures between 22–25 °C and 40–45% relative humidity, for further examinations. 2.3. Raman measurements and SERS analyses The SERS spectral measurements were conducted with a confocal Raman spectrometer (Horiba XploRA™ PLUS). The instrument, which included Raman spectrometer equipped with an Olympus research microscope, utilized diode lasers, which passed through focusing lens to sample surfaces. The resulting backscattered light was collected and collimated by the same focusing lens before being filtered and focused into the pinholes, finally to a thermoelectric-cooled CCD spectrometer. All the Raman and SERS characterizations were obtained using an excitation wavelength of 785 nm and a 10× objective lens providing a diameter laser spot around 100 μm on a characterized sample area. Using 600 lines/mm grating, the spectral range was obtained from 2500 to 100 cm−1. For a thorough investigation, each sample was analyzed by both conventional Raman spectroscopy (RS) and the proposed SERS technique. With the RS analyses, Raman signals were collected directly from the inks written on the paper, as illustrated in Fig. 2(Top (a)–(c)). For each measurement, a selected written ink area of each sample was visually located under the microscope and controlled by the motorized stage. From the selected ink area of equally spaced 150 × 150 μm2 square grid, a multiple-point acquisition for the total of nine Raman spectra was performed. With a constantly controlled 10 mW laser power output, each spectral data was acquired with three averaging for a total duration of 15 s. With the SERS analyses, Raman signals were collected after the written ink specimens were transferred onto the surface of the SERS substrates. As illustrated in Fig. 2(Bottom (a)–(f)), a simple ink transfer process began when a tiny piece of paper specimen with written ink line was punched at less than 2.0 mm2 area. The paper specimen was placed on top of the substrate center with the written side faced down. Afterward, equally mixed chloroform/methanol at 5 μL was dropped on the paper specimen. The solvent would seeped through the specimen, eventually allowing the written ink to spread over the SERS surface. Shortly after 10 s, the paper specimen was removed from the SERS substrate and the Raman measurements were performed immediately. The SERS spectra were collected at center and corner areas on the SERS substrate, based on the acquisition parameters identical to those of the previous RS measurements. 3. Results and discussion The details of brand names, pen models, ink colors, and ink types were summarized in Tables 1 and 2for blue and black pen samples, respectively. From Table 1, the blue pen samples were categorized as oil-based, gel-based, and and water-based at approximately 65%, 27%, and 8%, respectively. From Table 2, the black pen samples were categorized as gel-based, water-based, and oil-based of approximately 54%, 22%, and 24%, respectively. These numbers roughly represented general proportions of ballpoint pen inks in Thailand, and probably in Southeast Asia. Because of the different bases of the pen inks, the written inks on the same paper could suffer from non-uniform ink distribution effect. In order to suppress the effect, the RS and SERS spectra were taken from 9 different areas for each sample surface. The complete RS and SERS measurement results for all the pen samples were illustrated in Supplementary 1. Please be noted that few areas, e.g., at a center position from the SERS measurement grid for the 08 Faber castell gripx 5 ball pen, exhibited signal saturation from strong fluorescence at the detector during the spectral collections. Hence, such collected spectra were not included in the plot. From the results, we compared the spectral data and categorized the pen samples 3

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Fig. 2. Raman testing procedures. (Top) Raman spectroscopy: (a) pen ink written on paper, (b) select ink area of interest, (c) multiple-point Raman acquisitions. (Bottom) Surface-enhanced Raman spectroscopy: (a) pen ink written on paper, (b) ink area removal, (c) place sampling on SERS chip, (d) add a drop of diluting agent, (e) remove sampling, let it dry, and select area of interest, (f) multiple-point Raman acquisitions.

whether they were active/inactive toward the RS and/or the SERS technique. The pen inks, whose Raman spectra exhibited any observable Raman peaks that could be assigned to any vibration bands, we considered the spectral results as active in their RS/SERS measurements. However, when the Raman results exhibited only broad spectra, but major Raman peaks could not confidently be observed, we considered the results as inactive. For a clear interpretation, Fig. 3 illustrated representative results of the RS and SERS measured spectra for the selected pen samples. From the RS technique, the Raman spectral data were dominated by broad fluorescence backgrounds without active vibrational band from the ink samples. Such effects occurred in the majority of the pen samples as measured from the RS technique. However, from the SERS technique, the Raman spectral data clearly illustrated the clear fingerprints for the pen inks due to high signal enhancements and probably fluorescence quenching. With this approach, the pen samples could now be assigned into category for active/inactive RS and SERS, as included in Tables 1 and 2. From the conventional Raman spectroscopy, the results indicated that only 20% of the blue pens, and, interestingly, none of black pens, were RS-active. In this experiment, most of the pen inks were not RS-active because an NIR laser of 785 nm yielded a much lower Raman cross-section than a laser of visible wavelengths. On the other hand, by utilizing the SERS technique, the majority of both blue and black samples apparently showed highly spectral improvements. Only a few SERS-active samples, e.g., the pen number 33, 39, 40, and 45, delivered lower signals than the conventional RS. With the proposed SERS method, the number of the active pen samples yielding vibrational bands were significantly increased to 90.5% of total samples. Further considerations also proved no significant difference in percentage between the SERS-active blue and black samples. All Raman fingerprints of the RS and SERS spectra were finally classified based on vibrational peaks. Although forensic document investigations often use multiple techniques (e.g., visualization, chromatography, photoluminescence, etc.) to classify and discriminate the pen inks, they were beyond the scope of this study. Processes from which the Raman spectra were classified into different respective groups for each pen color, based on either the conventional Raman and SERS technique. Nine Raman spectra without any spectral treatments, i.e., background subtractions, normalization, smoothing as collected from different locations from a single sample, were therefore carefully examined and marked for 10 highest Raman intensity. Classifications of the pen groups were therefore assigned according to either blue or black pen color from the RS and SERS analyses. The classification processes were based on an assumption that the writing pens obtained from indiscriminate geographical locations may have contained similar base pigments but with different additives. We therefore looked for distinguishable Raman spectra from the assigned Raman bands across the collection of the pen samples of the same pen color and measurement technique. That is, if a collection of the pen samples had exactly the same Raman bands, they were classified into the same pen group. In Table 3, the results from the conventional RS measurements allowed classifications of black and blue pens into different groups. For the black pen samples, the RS results could only be categorized into one indistinguishable RS-inactive group, as evidently shown in Fig. 3 from fluorescent spectra with very weak Raman intensity. For the blue pens samples, on the other hand, their RS results were categorized into 6 groups. While a majority of which were RS-inactive, 10 out of 49 blue pens exhibited clearly differentiable Raman 4

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Table 1 Blue pens, Ink detail, maker, Raman and SERS (active/non active). Pen No.

Brand

Model

Color

Made In

Ink Type

Raman Active

SER Active

1 2 3 4 5 6 7 8 9

Aihao Allianz BlC Blue cross medium Blue ink Braza Elephant Faber castell Faber castel

Blue Blue Blue Blue Blue Blue Blue Blue Blue

China – Vietnam – Korea – – Malaysia Malaysia

Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based

Yes Yes No Yes No No No No No

Yes Yes Yes Yes Yes Yes Yes Yes Yes

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

G’soft Horse Lamy Lamy M$G Monami Paper mate Parker Parker Parker Quantum Quantum Quantum Quantum Rally thamasart Sapakacharthai Sheaffer Skate UD Yellow duck Yoya line Zebra

5271 – Super EZ 0113e Line friend 11 Drift Grip X5 Needle ball 1444 Hi-grip H-999 M16 M63 – FX zeta Renoid – Roller ball 0.8 mm ball pen Sapphire 1241 Signature 100 Geluloid 77 Gelluloid color – – – 300 – SGN – – Technoling

Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue

Thailand – Germany Germany China China – – – France India India – China Thailand – – – – – China India China

Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based Oil-based

No No No No No No No No No No No No No No No No No No No No No No No

Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No Yes Yes Yes Yes Yes Yes No

Pen No.

Brand

Model

Color

Made In

Ink Type

Raman Active

SER Active

33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49

Artline Drawing Faber castell Quantum Steadler Dong-A UD Uni Uni Uni Exam – oriented Maple Monami Muji Ohto Parker Pentel Steadler

Shachihate Ek-2305 Grip fine CD marker Permanent Lumocolor – Erasable Ball eye micro Ball micro deluxe UB – 155 Ball signo RT UMN – 105 – Kassimi Lris 2057 – Cansec free roller Quink 0.7 mm gel pen 1035 – c Luna

Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue Blue

– Malaysia Japan Germany – – Japan Japan Japan China – China Japan Japan France China China

Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based

Yes No No No No No Yes Yes Yes No No No Yes Yes No No Yes

Yes Yes No Yes Yes No Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes

gel gel gel gel gel gel gel gel gel gel gel gel gel

bands, and therefore were classified into 5 RS-active blue pen groups. Although some of the pen groups held similar Raman bands, the classification processes were solely focused on differentiations among their Raman spectra. For examples, the pen No. 41, 46, and 49, assigned with 680, 750, 1135, 1337, and 1525 cm−1, were classified into the blue pen’s Group 2, while the pen No. 39, and 40, assigned with only 680, 740, 1140, 1527 cm−1, were classified into Group 3. Although the instrument sensitivity of ±10 cm−1 was taken into account, the RS-active blue pen Group 3 did not belong in the RS-active blue pen Group 2, because their total spectral information were distinguishable. In Tables 4 and 5, similar classification processes have also been performed across all SERS spectral results from the blue and black pen samples, as observed from selected Raman spectra in Fig. 3. From Table 4, for examples, a number of the blue pens, assigned with 340, 420, 526, 616, 792, 914, 1180, 1370, 1587, and 1620 cm−1, were classified into the SERS-active blue pen’s Group 1. The pen No. 40, however, was classified into Group 6 because it had one clearly distinguishable band assignment at 727 cm−1 and therefore did not belong into Group 1. Therefore, the SERS analyses could confidently classify the blue pens into 13 clearly distinguishable groups. Similarly, from Table 5, the SERS analyses could confidently classify the black pens into 15 distinguishable 5

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Table 2 Black pens, Ink detail, maker, Raman and SERS (active/non active). Pen No.

Brand

Model

Color

Made In

Ink Type

Raman Active

SER Active

50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81

BIC BIC Cross Elephant Faber castell G'soft Lancer Lancer Paper mata Parker Pentel BIC Dong-A Marvy Monami Monami Pilot magic Schneidcer Stabilo Steadler Aihao Aihao Black micron Faber castell G'soft G'soft G'soft G'soft Gelly roll Horse Hybrid Hybrid

Oil pen Xtra EZ – – 0.7 mm GS007 – Spiral 825 – – – Boss Magic no.2950 – live color – – point 88 Sign pen – – – 0.4 mm Envy – BK Oil gel PDA – BK Twist –

Black Black Black Black Black Black Black Black Black Black Black Black Black Black Black Black Black Black Black Black Black Black Black Black Black Black Black Black Black Black Black Black

India Vietnam – India Malaysia Thailand Thailand – – – Japan China Korea Japan Korea Korea Korea Germany Germany Germany China China Japan Germany Thailand Thailand Thailand Thailand Japan – Japan Japan

Oil – based Oil – based Oil – based Oil – based Oil – based Oil – based Oil – based Oil – based Oil – based Oil – based Oil – based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based

No No No No No No No No No No No No No No No No No No No No No No No No No No No No No No No No

Yes Yes Yes No Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No

Gel grip Gel grip

gel gel gel gel gel gel gel gel gel gel gel

Pen No.

Brand

Model

Color

Made In

Ink Type

82 83 84 85 86 87 88 89 90 91 92 93 94 95

Magic gel Monami Paper mate Pentel Pentel Pentel Pentel Quantum Quantum Sarasa clip Stabilo Steadler Tous UD

– – – ener gel 0.5 mm ener gel 0.7 mm ener gel 1 mm KN 105 hybrid technical Geluloid 88 Geluloid Cr – – – T – 260 –

Black Black Black Black Black Black Black Black Black Black Black Black Black Black

Korea China – Japan Japan Japan – India India Japan Germany China China Korea

Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based Water-based

gel gel gel gel gel gel gel gel gel gel gel gel gel gel

Raman Active

SER Active

No No No No No No No No No No No No No No

Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No Yes Yes Yes

groups. The results obviously indicated that the proposed SERS technique promoted a wide variety of the vibrational bands from the pen inks, and therefore was very powerful tool to help differentiate the pen sample, when the conventional Raman spectroscopy have not been able to accomplish without complete Raman assignments from the respective pen ink databases. According to the previous study [10], parts of pigments contained in many RS- and SERS-active pen groups could be identified from the assigned vibrational bands. In brief details, the majority of the blue and black pens generally contain either pigment blue 15 (PB15) that belongs to the class of phthalocyanines, or the pigment violet 23 (PV23), also known as bluish violet, that belongs to the class of the oxazines, or a combination of both. Interestingly, some SERS-active samples, i.e., pen No. 12, 43, and 82, delivered different SERS fingerprint spectra between two measurement areas, i.e., the center and corner of the SERS substrate, because of a compositional separation from a thin-layer chromatography (TLC) effect on the nanostructured surfaces [27]. The TLC effect on the SERS substrate surfaces could be advantageous for ink analysis and was an open topic for future study. To determine the ability to distinguish the pen samples based on their RS and SERS characteristics, the discriminating powers (DP) were determined. According to Smalldon and Moffat [28], the DP value was an indication on the selectivity of the characterizing technique to differentiate samples, which in this study were a variety of the brand names, ink colors, and ink types of the ballpoint pens. The DP values were calculated by:

6

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Fig. 3. (a)–(d) Representative of Raman and SERS spectra of ballpoint pen samples. Table 3 Classified groups, Raman bands and calculated DP based on RS measurement. Conventional Raman Technique Pen Color

Groups

Raman Bands

Pen No.

No. of Pens

Indistinguishable pair

Blue

1 2 3 4 5 6 Total DP Groups 1 Total DP

750, 1329, 1540 680, 750, 1135, 1337, 1525 680, 740, 1140, 1527 1082 484, 590, 680, 750, 1142, 1337, 1452, 1525 Non Active

1, 2, 4 41, 46, 49 39, 40 45 33 The Rest

3 2 2 1 1 39

Raman Bands Non Active

Pen No. All

No. of Pens 46

3 1 1 0 0 741 748 0.36 Indistinguishable pair 1035 1035 0

Pen Color Black

DP =

Number of discriminated pairs , Number of possible pairs

And analyzed based on the RS and SERS results from both blue and black ink samples. For the blue pens, the total number of 49 samples equaled 1176 possible pairs. The RS-inactive blue pens yielded 748 indistinguishable pairs, as already reported in Table 3. We could therefore deduce that the RS technique allowed 428 discriminated pairs for the blue pens, corresponding to the calculated DP value at approximately 0.36. Such result however was much smaller compared to that at 0.68 obtained from the previous study for the blue pen inks [10]. It was most likely that the published report utilized a combination of two excitation laser wavelengths and considered the DP values only from the RS-active sample sets. On the other hand, our investigations included both the RS-active and inactive blue pen samples from one laser wavelength. When we considered the black pens for the total of 46 samples, none of them 7

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Table 4 Classified groups of blue pens, Raman bands and calculated DP based on SERS measurement. Surface Enhance Raman Scattering Technique Pen Color

Groups

Raman Bands

Pen No.

No. of Pens

Indistinguishable pair

Blue

1

340, 420, 526, 616, 792, 914, 1180, 1370, 1587, 1620 751, 1180, 1340, 1542 420, 616, 759, 1012, 1174, 1370, 1610 616, 1180, 1587, 1620 420, 759, 1174, 1370, 1610 340, 420, 526, 616, 727, 914, 1180, 1370, 1587, 1620 147, 225, 406, 452, 526, 1180, 1351, 1604 420, 616, 914, 1174, 1351, 1387, 1428, 1620 420, 529, 727, 801, 914, 1180, 1351, 1587, 1620 420, 616, 1174, 13,811,587, 1620 420, 616, 759, 920, 1174, 623, 1351, 1431, 1587 420, 920, 1174, 1620 Non Active

3, 5, 6, 7, 8, 9, 10, 11, 16, 18, 20, 21, 22, 23, 24, 26, 27, 29, 31 1, 2, 4, 12, 14, 15, 17, 19, 28, 30, 36 46, 49 44, 45 37, 48 40

19

171

11 2 2 2 1

55 1 1 1 0

13 43 42

1 1 1

0 0 0

41 47

1 1

0 0

34

1 6

0 15 244 0.79

2 3 4 5 6 7 8 9 10 11 12 13 Total DP

Table 5 Classified groups of black pens, Raman bands and calculated DP based on SERS measurement. Pen Color

Groups

Raman Bands

Pen No.

Black

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Total DP

340, 310, 494, 503, 212, 420, 480 311, 122, 557, 415, 415, 615, 415, 615,

50, 79, 62, 71, 74, 64, 72, 51 52 80 75 82 61 95 66

415, 615, 615, 615, 420, 525,

525, 615, 724, 807, 918, 1590, 1179, 1392, 1621, 1006, 1029, 1601 1077, 1145, 1218, 1264, 1406, 1590 1006, 1029, 1108, 1333, 1392, 1452, 1562, 1601 525, 1179, 1392, 1590, 1621 728, 918, 1179, 1392, 1590, 1621

615, 775, 1006, 1130, 1179, 1315, 1363, 1512, 1601, 1652 615, 1218, 1363, 1601 918, 1006, 1245, 1302, 1601 525, 615, 1006, 1145, 1179, 1384, 1590, 1621 643, 818, 858, 1139, 1264, 1363, 1497, 1590, 1601 1006, 1601 490, 614, 1179, 1245, 1333, 1571, 1601 1245, 1363

54, 83, 63, 85, 76, 84, 78

55, 88, 67, 86, 77 89,

56, 57, 58, 59, 60 91, 93, 94 68, 69 87 90

No. of Pens

Indistinguishable pair

8 6 5 4 3 3 2 1 1 1 1 1 1 1 1

28 15 10 6 3 3 1 0 0 0 0 0 0 0 0 66 0.94

was distinguishable by the RS characteristics. This resulted in zero discriminated pairs, and therefore yielded the DP value of zero for the black pens. The authors further investigated the DP calculations for the SERS investigations. The calculated DP value for the SERS-active blue pens was 0.79, which was significantly improved when compared to the result from previous work using colloidal SERS substrates (DP = 0.51) [18]. For the black pen samples, we obtained the DP value of 0.94, which was in good agreement with a reported result (DP = 0.90) [18]. The SERS technique was therefore very effective toward the discrimination of the pen samples, based on the differences in their chemical composition. Such technique, which readily enhanced the Raman signals, worked well for both ink colors and would be applicable to other commercial pen ink colors with similar medium contents. Based on calculation from our previous publication [29], our SERS substrate can detect dye molecules in range of few picogram. Therefore, our proposed SERS technique would allow confident collections of the pen ink databases and ensure the potential applications in forensic investigations of questioned documents in the near future. 4. Conclusions In this work, we reported the systematic study of reproducible thin-film based SERS substrates, prepared by a dc magnetron sputtering, for classifications of the ballpoint pen inks on the written documents. The pen samples used in this study consists of 49 blue and 46 black ballpoint pens, randomly collected from the local sources in Thailand. These pen samples were written on the sheet 8

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of paper and analyzed with both conventional RS and SERS techniques. The Raman measurements were performed with the confocal Raman spectrometer based on the 10× objective lens, the 785 nm excitation wavelength, and approximately 10 mW laser output. All of the Raman spectra for the blue and black pen samples were categorized into either RS or SERS active/inactive results, and could be classified into different distinguishable pen groups. These results were further calculated and analyzed as discriminating powers (DP). The RS method yielded the DP values at 0.36 and 0.0 for the blue and black pens, respectively, for distinguishing the ballpoint pens. From our proposed SERS technique, the majority of both blue and black samples showed highly improved spectral intensities with the increased DP to 0.79 and 0.94 for the blue and black pen samples, respectively. 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