Development of Well-Preserved, Substrate-Versatile Latent

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Development of Well Preserved, Substrate-Versatile Latent Fingerprints by Aggregation Induced Enhanced Emission Active Conjugated Polyelectrolyte Akhtar Hussain Malik, Anamika Kalita, and Parameswar Krishnan Iyer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13390 • Publication Date (Web): 04 Oct 2017 Downloaded from http://pubs.acs.org on October 5, 2017

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ACS Applied Materials & Interfaces

Development of Well Preserved, Substrate-Versatile Latent Fingerprints by Aggregation Induced Enhanced Emission Active Conjugated Polyelectrolyte Akhtar Hussain Malik,a Anamika Kalita,b and Parameswar Krishnan Iyer*a,b a

Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati-781039,

Assam. India. b

Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati-781039,

Assam. India. E-Mail: [email protected] KEYWORDS Polyelectrolyte, Latent fingerprints, AIEE, Third level details, Abrasion test.

ACS Paragon Plus Environment


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ABSTRACT The development of highly efficient latent fingerprint (LFP) technology remains extremely vital for forensic and criminal investigations. In this contribution, a straightforward, rapid and cost-effective method has been established for the quick development of well-preserved latent fingerprint on multiple substrates including plastic, glass, aluminium foil, metallic surfaces, etc. without any additional treatment, based on aggregation induced enhanced emission (AIEE) active conjugated polyelectrolyte (CPE) PFTPEBT-MI, revealing clearly the third-level details (ridges, bifurcations and pores) with high selectivity, high contrast and no background interference even by blood stains, confirming the ability of the proposed technique for LFPs detection with high resolution. The LFP development process was accomplished simply by immersing fingerprint loaded substrate into the CPE solution for ~1 minute followed by shaking off the residual polymer solution and then dried in air. The CPE were readily transferred to the LFPs because of the strong electrostatic and hydrophobic interaction between the CPE molecules and the fingerprint components revealing distinct fluorescent images on various smooth non-porous surfaces. INTRODUCTION Latent fingerprints (LFPs) detected at the site of crime are considered as imperative physical circumstantial evidence in forensic investigation and criminal justice1-4 because of the uniqueness in the ridge pattern for each individual and thus are unique and vital in cracking any criminal case and accurately identify the involvement of a suspect. Additionally, it continues to serve many other purposes in our day-to-day life such as safety inspection, access control, biometric proof, etc.5 Predominantly, LFPs if found are not easily visible to naked eye under ambient light, hence, additional tedious efforts are required for enhanced visualization and confirmation. Improvements in existing methods and techniques have been attempted for the identification of LFPs which include ninhydrin spraying,6 powder dusting,7


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iodine/cyanoacrylate fuming,8-10 and fluorescent dye staining.11 Although these methods were extensively used under specific circumstances, especially powder dusting technique which is presently operative for forensic investigation, but only under ordinary circumstances, it faces multiple challenges such as low contrast due to the non-fluorescence of the powders, that could damage the fingerprint details easily while brushing etc. Furthermore, few other above mentioned methods have similar limitations, which primarily are very low sensitivity, low contrast, easy damaging of fingerprint details, require longer processing time, post-treatments to develop, high background interference due to complex color patterns of the substrates etc. Therefore, development of simple, highly sensitive, rapid, cost-effective and reliable technique with low background interference for LFPs on multiple surfaces is highly desirable and remains in foremost demand as de facto proof and a challenge in the field of forensic research. Conjugated polyelectrolytes (CPEs) consisting of a hydrophobic π backbone possess unique optical properties, which along with hydrophilic ionic side groups, enhance their solubility in aqueous solution as well as makes it liable to exhibit strong electrostatic interactions with various charged species like proteins, amino acids, surfactants, etc. Thus, CPEs have emerged as key materials in the fields of biological sensors and bioimaging entities because of their excellent biocompatibility, low cytotoxicity, strong emission brightness and resistance to photo bleaching.12-23 Various efforts by researchers in the development of new fluorescent reagents and methods for developing LFP, more efficiently have appeared.24-40 These include exploring the aggregation-induced emission (AIE) approach for better visualization of LFPs using AIE-active molecules.32,33,37 Furthermore, conjugated polymer nanoparticles for the effective and highly distinguishable imaging of LFPs are also reported.34,24,38 However, due to the lack of selectivity towards fingerprint components and involvement of other posttreatments reduces the developing efficiency and increases the cost of the technique



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proportionally. Considering the above beneficial properties, we perceived that CPEs could also be applied in the detection of LFPs because LFPs contain sweat comprising of charged biomolecules41-43 and sebum consisting of oily mixture of wax esters, triglycerides and cholesterol ester components as a combinatio44,45 so as to have both hydrophilic and lipophilic interactions possible with appropriately chosen CPE. Herein we, explored a unique strategy with particular emphasis on the conceptual novelty of utilizing a new copolymer comprising aggregation-induced emission enhancement (AIEE) active molecule based cationic CPE (PFTPEBT-MI) to interact with the hydrophilic and lipophilic components in LFPs specifically for comprehensible and high resolution visualization of LFPs on various surfaces without any post-treatment46,47 or fingerprint transfer process24,48 for their development. Thus, PFTPEBT-MI could be applied practically on various substrates such as glass, aluminum foil, steel sheet, coins, adhesive tape (both transparent and colored) and OHP sheets by simply immersing the fingerprint laden substrate into aqueous PFTPEBT-MI solution for ~1 minute, withdrawing it, removing the residual solution and then drying it in air. The present approach is simple and has several advantages over previously reported methods. Firstly, the AIEE based CPE emits strongly on the solid substrate by avoiding the phenomenon of aggregation-caused quenching (ACQ) thus diminishing the background interference vis-à-vis enhancing contrast for LFPs. Secondly, no post-treatment or fingerprint transfer process is required for developing LFPs because of the presence of AIEE active moiety in the polymer backbone and polar head groups. Thirdly, the visualization of LFPs on the substrate preferentially depends upon the adhesion of the CPE to the ridges of fingerprints. Since this (PFTPEBT-MI) polymer is amphiphilic in nature, it can interact strongly with LFPs via both electrostatic and lyophilic interactions resulting in the selective binding with ridges rather than furrows which further enhances the contrast of fingerprints on


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various substrates. Finally, all the fingerprint patterns were very well preserved and retained high resolution even after applying external abrasion (chemical and physical) confirming the high reliability and robustness of this method. EXPERIMENTAL SECTION Materials and Instruments. All the chemicals were purchased from Aldrich, Merck (India) and were used as received. UV-visible and Fluorescence spectra were recorded on a Perkin Elmer Lambda-25 and Horiba Fluoromax-4 spectrofluorometer, respectively using 10 mm path length quartz cuvettes with a slit width of 3 nm at room temperature. Milli-Q water was used in all experiments. 1H NMR (600 MHz) and 13C NMR (100 MHz) spectra were obtained on Bruker Ascend 600 spectrometer and Varian respectively. Zetasizer Nano ZS90, Model No. ZEN3690, Malvern was used to calculate the zeta potential of the polymer. Gel Permeable Chromatography was carried out in THF using WATERS instrument taking polystyrene as standard. FESEM images were recorded on a Sigma Carl ZEISS scanning electron microscope. Different substrates used for depositing fingerprints, such as glass slides, adhesive tapes (colored and transparent), aluminum foil, steel plate, OHPC sheet, and metal coins were purchased from local market. The digital photographs of developed fingerprints were taken by Nikon D5200 camera under a hand held ultraviolet lamp (365 nm) purchased from sigma. Fluorescence microscope image was recorded on an Olympus BX51 microscope. Synthetic Procedure Synthesis of Monomer M2: 2, 7-Bis[9,9′-bis(6′′-bromohexyl)fluorenyl]-4,4,5,5-tetramethyl [1.3.2]dioxaborolane (M2) was prepared using established procedure.49 Synthesis of Monomer M3: M3 was prepared by a reported procedure.50 In short, 4Bromobenzophenone (1 g, 3.8 mmol) and zinc powder (0.74 g, 11.0 mmol) were added into a



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100 mL two-necked round bottomed flask. The flask was degassed and flushed with argon 23 times, after which anhydrous THF (40 mL) was injected. The mixture was cooled to −5 °C then titanium tetrachloride (0.60 mL, 5.7 mmol) was added gradually. The mixture was heated up to 25 °C and kept there for half an hour and then refluxed at 80 °C overnight. The reaction mixture was quenched with 10 % K2CO3 (aq.) and extracted with DCM. The organic layer was washed with water and dried over anhydrous sodium sulphate. The solvent was evaporated under reduced pressure and the residue was purified by silica gel column chromatography to get a white solid compound (1.31 g, 70 %). 1

H NMR (600 MHz, CDCl3, δ ppm): 7.29-7.23 (m, 4H; Ar H), 7.17-7.12 (m, 6H; Ar H),

7.03-7.00 (m, 4H; Ar H), 6.92-6.89 (m, 4H; Ar H). 13

C NMR (100 MHz, CDCl3, δ): 142.16, 140.11, 139.15, 132.78, 131.11, 130.98, 130.78,

127.70, 120.56. Synthesis of PFTPEBT: PFTPEBT was synthesized via Suzuki coupling polymerization as reported.49 M2 (0.100 g, 0.13 mmol), M3 (0.038 g, 0.08 mmol), 4,7-dibromo-2,1,3benzothiadiazole (0.015 g, 0.05 mmol), [Pd(PPh3)4] (0.007 g) and potassium carbonate (0.215 g, 1.5 mmol) were kept in a 50 mL two neck round-bottomed flask. A mixture of water and THF (1:3) was added to the flask using syringe and degassed thrice by flashing argon and freeze thaw cycle to remove any oxygen. The reaction mixture was then refluxed and stirred vigorously at 80 °C for 24h followed by repeated precipitation into excess of methanol 3-4 times. The precipitate was then filtered, washed with water/chloroform and dried under vacuum overnight to get the dark yellowish colored polymer (0.114 g, 75 %). 1

H NMR (600 MHz, CDCl3, δ ppm): 8.06 (b), 7.97-7.89 (b), 7.81 (b), 7.68 (b), 7.54-7.47 (b),

7.16-7.12 (b), 7.03 (b), 6.98 (b), 6.89 (b), 6.82 (b), 3.27 (b), 2.17-2.05 (b), 1.67 (b), 0.88 (b), 0.81 (b), 0.66 (b).


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Synthesis of PFTPEBT-MI: Brominated polymer PFTPEBT (0.05 g, 0.051 mmol) was taken into 25 mL round bottomed flask. To it 2 mL of 1-methyl imidazole was added and heated under stirring condition at 80º C for 24h. The whole reaction mixture was then poured into excess of chloroform kept in a separate 50 mL beaker and stirred for 2-3h to get precipitate. The light brownish precipitate was then washed several times with chloroform to remove trace quantities of methyl imidazole so as to obtain pure functionalized polymer PFTPEBT-MI (0.044 g, 75 %). 1

H NMR (600 MHz, CD3OD, δ ppm): 8.84 (b), 8.23 (b), 8.07-8.01 (b), 7.91 (b), 7.85-7.81

(b), 7.75 (b), 7.70 (b), 7.46 (b), 7.12 (b), 4.04 (b), 3.83 (b), 2.15-2.09 (b), 1.65 (b), 1.16-1.09 (b), 0.83-0.59 (b). Preparation of Stock Solutions Stock solution of PFTPEBT-MI was prepared in Milli-Q water at concentrations of 10 mM. The absorption and fluorescence measurements of the PFTPEBT-MI were carried out in a 3 mL quartz cuvette (1 cm × 1 cm) containing 33 µM of PFTPEBT-MI. Quantum yield calculations Photoluminescence quantum yield (Φs) of PFTPEBT-MI were determined using quinine sulphate (Φr = 0.54 in 0.1 M H2SO4) as standard from the equation shown below Φs= Φr (ArFs/AsFr)(ηs2/ηr2)



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Scheme 1. Synthesis of PFTPEBT-MI (a) 1,6-dibromohexane, 50% aq. NaOH, TBAI, 70 ºC, 4h. (b) bis(pinacolato)diborane, [Pd(dppf)Cl2], KOAc, dioxane, 85 °C, 12h. (c) TiCl4, Zn powder, THF, reflux, 12h. (d) Tetrakistriphenylphosphine palladium (0), 2M K2CO3 (aq.), THF, reflux, 24h. (e) PFTPEBT, 1-methyl imidazole, reflux, 24h. Where, r and s represent reference and sample, Φ denotes the quantum yield, A signifies absorbance, F shows relative integrated fluorescence intensity and η implies the refractive index of the medium. LFPs Collection Development and Visualization Prior to fingerprint deposition on various substrates, the donor (29 years old male) was asked to wash hands thoroughly with soap and rinse with water and ethanol then blown dry.


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Fingerprints were obtained by gently rubbing the fingertips across nose and forehead and then pressing the left hand thumb onto different surfaces with minimum pressure. The fingerprint laden substrates were then brought in contact with the developing polymer solution (0.2 mM) by directly immersing it into the petri dish for ~1 minute and then the substrate was withdrawn, smashed in air to splash off the excess solution and then air dried. For practical applications polymer solution was added into a spray bottle, together with 20%30% (v/v) of methanol to hasten water evaporation, and then sprayed onto the substrate. Photographs of the developed fingerprints were taken under UV lamp (365 nm) using Nikon D5200 digital camera with basic lens 18-55 mm. Abrasion Tests Physical abrasion test was performed by mounting double sided adhesive tape on to the substrate containing developed fingerprints and then peeling off the tape from the surface while chemical abrasions were done by treating the developed fingerprint substrates with various solvents like chloroform, acetone, THF, and toluene for 5-10 mins and the digital images were taken under UV lamp (365 nm) before and after abrasion. RESULTS AND DISCUSSION Synthesis and Characterization of Polymer PFTPEBT-MI. With an aim to develop highly efficient latent fingerprint technology that would advance the existing method used in forensic investigation a polymer probe with high brightness and large stokes shift that can offer superior contrast for the visualization of LFPs was essential. The synthetic procedure for the newly synthesized cationic polymer PFTPEBT-MI is shown in Scheme 1. Monomers M2, M3 and 4, 7-dibromo-2, 1, 3-benzothiadiazole was taken in the ratio of 0.5:0.3:0.2 and polymerized by Suzuki cross-coupling polymerization reaction to yield PFTPEBT. N-methyl imidazole was then introduced onto the side chain of PFTPEBT via post functionalization



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method to obtain cationic polymer PFTPEBT-MI in good yields without any hectic purification methods. PFTPEBT-MI (Figure 1a) was found to be soluble in polar solvents like water, dimethyl sulfoxide, methanol etc. All the products were characterized by 1H and

13

C

NMR spectroscopy (Figure S1-S4). The molecular weight (Mw) of PFTPEBT was found to be 7828 gmol-1 with a PDI of 1.17 (Figure S5).

Figure 1. (a) Structure of polymer 3,3'-((2-(4-(1,2-diphenyl-2-(p-tolyl)vinyl)phenyl)-7-(7methylbenzo[c][1,2,5]thiadiazol-4-yl)-9H-fluorene-9,9-diyl)bis(hexane-6,1-diyl))bis(1methyl-1H-imidazol-3-ium) bromide (PFTPEBT-MI). (b) Absorption and fluorescence spectra of PFTPEBT-MI in water. Inset: Color of polymer in solid and liquid state under the irradiation of UV lamp (365 nm). (c) Fluorescence spectra of the PFTPEBT-MI (33 µM) in water and water-THF mixture with varying fractions of THF. (d) Plot of fluorescence intensity against fraction of THF. Inset: Digital photograph of PFTPEBT-MI in water/THF mixtures with different THF fractions from 0 % to 90 % (in vol %) at 365 nm UV illumination.


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The zeta potential of PFTPEBT-MI was calculated to be 33 mV±1.52 at 25 ºC in water (Figure S6). The polymer PFTPEBT-MI shows absorption maxima at 350 nm and 460 nm in aqueous solution as well as in solid state (as film). PFTPEBT-MI displays emission maxima at 555 nm and 565 nm (λex=450 nm) in liquid (aqueous solution) and solid state (as film) respectively with stokes shift of ~100 nm. PFTPEBT-MI exhibits a molar extinction coefficient value of (ε) 35704 M-1 cm-1 and 16769 M-1 cm-1 at 350 nm and 460 nm respectively. The quantum yield (ɸ) of water soluble conjugated polymer PFTPEBT-MI was calculated to be 24 %. PFTPEBT-MI appeared dark yellowish colored in concentrated aqueous solution as well as in thin film state under the illumination of UV light (365 nm) (Figure 1b). To explore the AIEE character of the polymer, its emission studies were recorded in waterTHF mixture as shown in Figure 1c and 1d. It is clear that the fluorescence intensity of this polymer is less in pure water (good solvent), the intensity suddenly increases when THF (poor solvent) fraction (ƒTHF) was 30 % and the fluorescence intensity remains constant on further increasing the ƒTHF up to 90 %, which is further confirmed by the appearance of bright fluorescence under the irradiation of 365 nm UV lamp as shown in the inset of Figure 1d. The enhancement in the fluorescence intensity with increase in ƒTHF is due to the formation of polymer aggregates that endows the desired intense emission. Development of Latent Fingerprints and Digital Magnification LFPs were prepared from sebum and sweat components together by rubbing the finger with nose and forehead and depositing the fingerprints initially on four different substrates such as aluminium foil, glass slide, adhesive tape and coins as mentioned in experimental section. Fluorescence image of the developed fingerprints displayed yellowish green color under UV radiation (365 nm) with apparently higher contrast between the fluorescent ridges and



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Figure 2. Fluorescence images of LFPs developed with PFTPEBT-MI on various substrates such as aluminum foil, glass slide, adhesive tape and coin. non-fluorescent furrow without any post-treatment method which could be clearly distinguished on various substrates having different background colors by naked eye (Figure 2). The appearance of bright and quality fluorescence images of LFPs is due to the preferential increase in the local concentration of PFTPEBT-MI on the fingerprint ridges rather than on the furrows as observed in fluorescence microscope and FESEM images (Figure 3) by the process of adhesion, which may be attributed to both hydrophobic and electrostatic interactions between PFTPEBT-MI and the chemical components of the fingerprints.51 The hydrophobic interactions arise from the interactions between the fatty components (wax esters, fatty acids, squalene, and cholesterol) of the LFPs and the conjugated backbone of PFTPEBT-MI which contains AIEE active moiety as well, whereas the electrostatic interactions ascend from the sweat components (amino acids, proteins, glucose, etc.) and the terminal functional groups of PFTPEBT-MI. Using this PFTPEBT-MI for developing, LFPs with high resolution were obtained with distinct level 1-3 details, without any additional optical, thermal or chemical post-treatment, which is a prerequisite for definite identification of a person. Usually fingerprints are characterized at three levels of details.52 Level 1 detail is not unique enough for recognition and it includes cores and deltas. Level 2 includes ridge ending and bifurcation, which are the most characteristic features.


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Figure 3. (a) FESEM images of developed LFPs showing adhesion of CPE on (b) ridges rather than on (c) furrows with a lower and higher magnification. (d) Fluorescence microscopic images of the developed LFPs. Level 3 provides quantitative data for exact fingerprint recognition and it includes ridge path deviations, sweat pores and edge contours. The level 3 details of the fingerprint, features as a representative example of one developed in Figure 4a (left) and the regional magnified images are shown in Figure 4b (right) which include core (level 1), ridge ending, bifurcation and island (level 2) as well as pore (level 3), thus clearly providing prima facie evidence to prove the identification of an individual without contradiction and validating the practicability of the present system for the LFP recognition. To further investigate whether the external abrasions can destroy or alter the developing fingerprints, physical abrasion test (Figure S7) was performed as mentioned in the experimental section to the developed fingerprints on various substrates such as aluminium foil, glass slide, adhesive tape (transparent), steel plate, OHP sheet, coins, and adhesive tape (blue color) and digital images were captured before and after abrasion under 365 nm UV lamp illumination (Figure 5). Though in some cases the fluorescent images of the fingerprints



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Figure 4. (a) High-resolution photographs of latent fingerprints after developing with PFTPEBT-MI (b) showing level 1-3 details including island, bifurcation, core, ridge ending, and pores under the excitation of UV illumination (365 nm). To further investigate whether the external abrasions can destroy or alter the developing fingerprints, physical abrasion test (Figure S7) was performed as mentioned in the experimental section to the developed fingerprints on various substrates such as aluminium foil, glass slide, adhesive tape (transparent), steel plate, OHP sheet, coins, and adhesive tape (blue color) and digital images were captured before and after abrasion under 365 nm UV lamp illumination (Figure 5). Though in some cases the fluorescent images of the fingerprints captured were scratched to some degree, nevertheless, all the fingerprint patterns were very clear even after applying external abrasion. Similarly, chemical abrasion test was performed by treating the developed LFP substrate such as glass slide with various chemicals such as THF, chloroform, acetone and toluene (Figure S8), but no such disruptive interference was observed. Therefore, this strategy was effective in developing LFP on various substrates with negligible effect even in the presence of intense external interferences.


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Figure 5.

Digital photographs of latent fingerprints developed with PFTPEBT-MI on

different substrates before and after abrasion with double sided tape under UV irradiation (365 nm).

Figure 6. Blood laden LFP developed on a glass slide showing no interference even by biological sample. Furthermore, we also developed blood laden fingerprints as shown in Figure 6. It is clear that even the blood stained fingerprints can be developed easily without any interference from blood and its components.



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Application of PFTPEBT-MI for LFP Development To evaluate the practicability of PFTPEBT-MI assay for the detection of LFPs as the “weight of evidence” in real forensic samples, the CPE solution was added into a spray bottle, along with 20%−30% (v/v) of methanol for quick evaporation of residual polymer solution and then sprayed onto the substrate such as drinking glass or cups and soft drink (beverage) tin can to develop the LFPs. As shown in Figure 7, the fluorescence generated by CPE in the fingerprint patterns were of high-resolution and are strong evidence with 1-3 clarity levels. Although the blank ink printed on the beverage tin can interfere with the visualization of LFPs, yet the fingerprint details could be recognized very clearly with high resolution and high contrast. Similar high quality results could also be observed distinctly on a drinking glass (Figure 7).

Figure 7. Images of LFPs developed with PFTPEBT-MI (a) tin can and (b) drinking glass. The left and middle set of each image represent the photographs of latent fingerprints under 365 nm light irradiation. The right set shows the enlarged fluorescence images of latent fingerprints with third level details.


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A comparative study was also carried out to demonstrate the advantage of the current PFTPEBT-MI platform over the previously reported systems (Supporting Information Table S1). As highlighted in the Table S1 (Supporting Information), the PFTPEBT-MI AIEEpolyelectrolyte system works rapidly and with highest clarity resolution (level 3) since it has the ability to bind components as a combination of both hydrophilic and lipophilic interactions, can emit brightly in solid state (aggregated form-AIEE) or in the presence of organic molecules, is highly resistant to external abrasions and foremost no post treatment of temperature spraying/coating the CPE over the finger prints. All other methods and materials reported in literature need a combination of materials, additionally developing materials, post treatment either in the form of high temperature heating or UV-light or infra-red light and with surfactants yet level 3 details are not visible with these techniques. In several cases heating and UV irradiation was also necessary that destroyed the surface and pores were buried, rendering these methods of little practical use. With such multiple advantages that we could achieve with the new AIEE-polyelectrolyte, very high resolution LFPs on practically multiple surfaces could be developed rapidly thereby introducing a very versatile method for LFP identification. CONCLUSION In conclusion, a simple, sensitive and cost-effective approach for the enhanced visualization of latent fingerprint based on AIEE active conjugated polyelectrolyte PFTPEBT-MI was successfully developed by simply dipping the fingerprint laden substrate into the polymeric solution or spraying the polymer on the surface of evidence. The high fluorescence brightness and large Stokes shift contributed to the high-resolution imaging of latent fingerprints without any post-treatment such as fume treatment, surfactants, etc. for latent fingerprint development, therefore providing highly reliable and concrete evidence (1-3 level), which makes the identification of an individual suspect easy. Thus, PFTPEBT-MI polymer could be



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effectively applied in developing LFPs with high-sensitivity on practically any surface such as glass, aluminum foil, steel, coins, adhesive tape etc. with negligible interference including blood, in the presence of even harsh external abrasions which could hinder in the identification of personal information, thereby demonstrating successfully the durability and unmatched ability of this approach. We anticipate that the present system could be used in the detection of LFP and further find its utility for various forensic and biometric applications. ASSOCIATED CONTENT

Supporting Information. Characterization spectra of synthesized molecules and polymer, Zeta potential graph of PFTPEBT-MI, image of chemical abrasion test and comparison table. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] FAX: +91 361 258 2349 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT Financial support from Department of Information Technology, DeitY Project no. 5(9)/2012NANO

(Vol.II),

Department

of

Science

and

Technology

(DST),

India

(Nos.

DST/TSG/PT/2009/23 and DST/SERB/EMR/2014/000034), DST-Max Planck Society, Germany (IGSTC/MPG/PG(PKI)/2011A/48) are gratefully acknowledged. The CIF and Department of Chemistry, IIT Guwahati are acknowledged for instrument facilities. REFERENCES (1)

Faulds, H. On the Skin-furrows of the Hand. Nature 1880, 22, 605.


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(2)

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Development of Well-Preserved, Substrate-Versatile Latent

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