Hydrochromic Approaches to Mapping Human Sweat Pores

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Hydrochromic Approaches to Mapping Human Sweat Pores Dong-Hoon Park,† Bum Jun Park,*,‡ and Jong-Man Kim*,†,§ †

Department of Chemical Engineering, Hanyang University, Seoul 04763, South Korea Department of Chemical Engineering, Kyung Hee University, Yongin 17104, South Korea § Institute of Nano Science and Technology, Hanyang University, Seoul 04763, South Korea ‡

CONSPECTUS: Hydrochromic materials, which undergo changes in their light absorption and/or emission properties in response to water, have been extensively investigated as humidity sensors. Recent advances in the design of these materials have led to novel applications, including monitoring the water content of organic solvents, water-jet-based rewritable printing on paper, and hydrochromic mapping of human sweat pores. Our interest in this area has focused on the design of hydrochromic materials for human sweat pore mapping. We recognized that materials appropriate for this purpose must have balanced sensitivities to water. Specifically, while they should not undergo light absorption and/or emission transitions under ambient moisture conditions, the materials must have sufficiently high hydrochromic sensitivities that they display responses to water secreted from human sweat pores. In this Account, we describe investigations that we have carried out to develop hydrochromic substances that are suitable for human sweat pore mapping. Polydiacetylenes (PDAs) have been extensively investigated as sensor matrices because of their stimulusresponsive color change property. We found that incorporation of headgroups composed of hygroscopic ions such as cesium or rubidium and carboxylate counterions enables PDAs to undergo a blue-to-red colorimetric transition as well as a fluorescence turn-on response to water. Very intriguingly, the small quantities of water secreted from human sweat pores were found to be sufficient to trigger fluorescence turn-on responses of the hydrochromic PDAs, allowing precise mapping of human sweat pores. Since the hygroscopic ion-containing PDAs developed in the initial stage display a colorimetric transition under ambient conditions that exist during humid summer periods, a new system was designed. A PDA containing an imidazolium ion was found to be stable under all ambient conditions and showed temperature-dependent hydrochromism corresponding to a colorimetric change near body temperature. This feature enables the use of this technique to generate high-quality images of sweat pores. This Account also focuses on the results of the most recent phase of this investigation, which led to the development of a simple yet efficient and reliable technique for sweat pore mapping. The method utilizes a hydrophilic polymer composite film containing fluorescein, a commercially available dye that undergoes a fluorometric response as a result of water-dependent interconversion between its ring-closed spirolactone (nonfluorescent) and ring-opened fluorone (fluorescent) forms. Surfacemodified carbon nanodots (CDs) have also been found to be efficient for hydrochromic mapping of human sweat pores. The results discovered by Lou et al. [Adv. Mater. 2015, 27, 1389] are also included in this Account. Sweat pore maps obtained from fingertips using these materials were found to be useful for fingerprint analysis. In addition, this hydrochromism-based approach is sufficiently sensitive to enable differentiation between sweat-secreting active pores and inactive pores. As a result, the techniques can be applied to clinical diagnosis of malfunctioning sweat pores. The directions that future research in this area will follow are also discussed.

1. INTRODUCTION Hydrochromic materials (including conjugated organic molecules), which undergo colorimetric transitions upon interaction with water, have been extensively investigated in the context of humidity sensors.1−9 Polymer films that have periodically ordered structures have also been used for humidity sensing because they have reflectance properties that depend on the relative humidity (Figure 1a).3−6 Hydrochromic materials have recently been employed for the detection of water in organic solvents.10−13 For instance, a substance containing an anthracene fluorophore coupled with an amino acid group was utilized to detect water in organic solvents over a wide concentration range. The presence of water in organic solvents enhances the intensity of the fluorescence emission of this © XXXX American Chemical Society

substance by suppressing photoinduced electron transfer (PET) (Figure 1b).10 More recently, a new type of hydrochromic material containing a chemically modified hydrochromic oxazolidine dye has been described. The dye, which is colorless under anhydrous conditions, undergoes a waterpromoted transition to blue (Figure 1c).14 Color generation in this substance is attributed to a ring-opening reaction of the oxazolidine moiety in the presence of water. It is notable that hydrochromic-dye-coated paper undergoes the colorimetric response reversibly over repetitive hydrating and drying cycles. As a result, the dye serves as an erasable ink. Received: March 10, 2016

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DOI: 10.1021/acs.accounts.6b00128 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. Applications of hydrochromic materials from (a) humidity sensing and (b) water content detection to (c) recently uncovered reusable paper and (d) human sweat pore mapping. Reproduced with permission from refs 6, 10, and 14. Copyright 2011 Royal Society of Chemistry, 2010 Royal Society of Chemistry, and 2014 Nature Publishing Group, respectively.

Figure 2. Schematic illustration of the formation of supramolecular polydiacetylenes from self-assembled diacetylenes. Reproduced with permission from ref 15. Copyright 2014 Nature Publishing Group.

be accomplished using conventional scanner-type methods. Moreover, the new sweat pore mapping method based on hydrochromic materials is not limited to fingertips. In principle, the technique can be applied to analysis of other sweatsecreting areas of human skin including the palm, foot, chin, cheek, and back. Because malfunctioning sweat pores might be correlated with certain diseases, mapping of sweat-secreting active pores could be clinically important. In addition, cosmetic companies are interested in sweat pore mapping technologies because information about the distribution of active sweat pores might be useful in developing skin care products. The results of our recent studies aimed at the development of hydrochromic materials for human sweat pore mapping are discussed in this Account. The discovery of hydrochromic polydiacetylenes (PDAs) and their optimization and applica-

In a recent study leading to a new application of hydrochromic materials, we observed that softly pressing a fingertip on a film coated with a hydrochromic material results in the generation of a fluorescent microdot pattern that matches that of sweat pores on the fingertip (Figure 1d).15−17 This finding, which enables precise mapping of sweat pores on a fingertip, is significant because it suggests that a new method exists for fingerprint recognition that can serve as an alternative to the conventional technique, which is dependent on the analysis of ridge patterns.18−28 Specifically, human sweat pore patterns located along the friction ridge are also unique, and consequently, they can be utilized for identification of individuals.24−28 In addition, hydrochromic-material-based sweat pore mapping enables differentiation between sweatsecreting active pores and inactive pores, something that cannot B


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Figure 3. Hydrochromic properties of PDA containing a hygroscopic element (Cs). (a) Schematic of the procedure for fabrication of a poly(PCDA−Cs)-coated PET film (PDA-Cs film). (b) Colorimetric response of a PDA-Cs film as a consequence of hydration. (c) Corresponding SEM images of the PDA-Cs film showing a morphological change. (d) XRD patterns of the PDA-Cs film before (black line) and after (red line) exposure to water. (e) Schematic of the water-promoted phase transition of the PDA. Reproduced with permission from ref 15. Copyright 2014 Nature Publishing Group.

moieties as headgroups would make the initially formed blue state of PDAs highly sensitive to water and, as a result, would enable the PDAs to undergo colorimetric transitions upon hydration. Specifically, we reasoned that the residual hygroscopic headgroup containing DA monomers in the PDA would dissolve in water, to create a void in the blue-phase PDA. Moreover, we believed that because of its hygroscopic nature, the blue-phase PDA would have the potential to undergo a blue-to-red phase (color) change in the presence of water. In order to test the above proposal, a PDA film containing hygroscopic head groups composed of cesium carboxylate (PDA-Cs) was prepared. The DA monomer used for this purpose was generated simply by mixing a tetrahydrofuran solution of 10,12-pentacosadiynoic acid (PCDA) with an aqueous cesium hydroxide solution under ambient conditions (Figure 3a).15 The resulting colorless PCDA−Cs complex was spin-coated onto a solid substrate (i.e., poly(ethylene terephthalate), PET). Then the colorless PCDA-Cs film on the PET substrate was subjected to UV irradiation (254 nm, 1 mW/cm2, 30 s), which induced photopolymerization to form the blue-colored PDA film. Importantly, the PDA-Cs film produced in this manner is sensitive to water. For example, a soft blow of human expiration causes it to undergo an immediate color change from blue to red (Figure 3b). Using scanning electron microscopy (SEM) (Figure 3c), we demonstrated that exposure of the film to water

tion to human sweat pore mapping are described in detail. In addition, our most recently developed fluorescein-based hydrochromic sweat pore mapping strategy and the surfacemodified carbon nanodot (CD)-based approach developed by Lou et al.29 are also described.

2. DISCOVERY OF HYDROCHROMIC POLYDIACETYLENES PDAs are structurally and optically unique polymers that possess main-chain backbones comprising alternating conjugated double and triple bonds.30−39 The unique structure and electronic features of PDAs arise as a consequence of their common method of preparation through polymerization (UV, heat, or γ ray) of self-assembled diacetylene (DA) monomers (Figure 2). Because they contain extensively delocalized πelectron networks, the initially formed PDAs often exist in a blue-colored form. Interestingly, a blue-to-red color transition can be triggered by various stimuli, including volatile organic compounds (VOCs),40,41 surfactants,42,43 ligand−receptor binding,44−47 heat,48 and mechanical stress.49,50 Although the blue-to-red color change property of PDAs has been extensively investigated, the use of water to promote the colorimetric transition had not been explored prior to the first investigation carried out by our group.15 At the outset of our efforts, we envisioned that incorporation of hygroscopic C


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Figure 4. Sweat pore mapping on a PDA-Cs film. (a, b) Snapshot of a fingertip (a) and a digitally scanned fingerprint image (b). (c) Schematic cross section of the skin. (d, e) Fluorescent image (excitation at 510−550 nm) of a fingerprint deposited on a PDA-Cs film (d) and a magnified image of the marked area (e). (f) Superimposition of the fluorescence image on the scanned image in (b). Fluorescent dots coincide with the black dots that correspond to the positions of sweat pores, whereas the black dots indicated by blue circles show pores that do not secrete sweat. Reproduced with permission from ref 15. Copyright 2014 Nature Publishing Group.

sweat pore pattern with a digitally scanned fingertip image demonstrates that the black-colored sweat pore positions precisely match the positions of the sweat-promoted redcolored dots (Figure 4f). It should be noted that the blackcolored dots inside the blue circles correspond to inactive sweat pores (i.e., ones that do not secrete sweat). This remarkable discovery enables visual differentiation between sweat-secreting active pores and malfunctioning inactive pores. A full fluorescence image of sweat pores on a fingertip is displayed in Figure 5.52 The sweat pore mapping system, which takes advantage of the hydrochromic property of a PDA-Cs film, was used as a method for latent fingerprint analysis.15 For this purpose, a sweat pore fingerprint fluorescence pattern was produced by lightly pressing a fingertip on a PDA-Cs-coated PET film. A latent sweat pore image on conventional paper was generated from the finger of the same individual using a modified ninhydrin staining method. Image analysis software (ImageJ) was used to determine the position of each sweat pore in both images (Figure 6a,b; the detailed procedure is given in ref 15), and then a pore-pattern matching program implemented in MATLAB was employed to compare the two independently generated sweat pore fingerprint patterns. The fluorescence microscopy and latent fingerprint images are displayed in Figure 6c,d, where the blue-colored dots are points corresponding to sweat pores in the two images that are located at the same position. The correspondence between the two sweat pore patterns, which can be seen by viewing the two image in Figures 6c,d, clearly demonstrates the reliability of the new sweat pore mapping technique and the validity of the porepattern-matching protocol (Figure 6e−g).

induces a morphological change from strips to isolated aggregate structures. X-ray diffraction (XRD) analysis of PDA-Cs (Figure 3d) provided evidence to support the proposal that its hydrochromic property is associated with structural deformation of the π-conjugated PDA array. Accordingly, using XRD analysis and Bragg’s law, we found that the interlamellar distances of the intact PDA-Cs film in its blue- and red-colored forms are 4.81 and 6.19 nm, respectively. The significant increase that takes place in the interchain distance in the PDA lamellar structures demonstrates that water causes partial distortion of the highly conjugated main-chain backbone of the PDA that is responsible for the solvatochromic behavior (Figure 3e).

3. HUMAN SWEAT PORE MAPPING USING A HYDROCHROMIC POLYDIACETYLENE Friction ridge patterns on human fingertips are unique, permanent, and immutable. As a result, these patterns have been used for identification of individuals in criminal investigations, for license validation, and in biometric security applications.18−28 Sweat pores (20−40) located on the friction ridges of fingertips (Figure 4a−c) can also be utilized for personal identification because they are arranged in patterns that are unique to each individual.51 We demonstrated that the hydrochromic property of the PDA-Cs-coated PET film can be employed advantageously in human sweat pore mapping.15 Notably, a tiny amount of water secreted from sweat pores when a fingerprint is deposited on the blue-phase PDA-Cs film causes a blue-to-red color change and production of an associated fluorescence emission pattern. As can be seen in the fluorescence microscopy images (Figure 4d,e), the red-colored light-emitting dots correspond to sweat pores on the fingerprint. Visual inspection of the overlap of a fluorescence D


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would enable the fabrication of cheap, user-friendly, ready-touse, and disposable sensor systems. To evaluate the compatibility of PDA-based hydrochromic materials with inkjet printing, PDA-Cs was deposited on paper. However, no noticeable color transition occurs when a drop of water is placed on the PDA-Cs-coated paper. The lack of response is a consequence of the hydrophobic surface property of the PDACs-coated paper (i.e., the water contact angle θ is ca. 109°), which suppresses absorption of water and consequently prevents a hydrochromic response. In contrast, the water contact angle of PDA-Cs-coated PET film is ca. 60°, and as a result, the hydrophilic surface of the film allows water penetration that leads to the color change described earlier. On the basis of the results of FTIR analyses, the decrease in affinity for water when PDA-Cs is coated on paper appears to be associated with the transformation of cesium carboxylate moieties to more non-hydrochromic hydrophobic species.16 It was demonstrated recently that amphiphilic DA monomers containing an imidazolium salt headgroup (DA-1; Figure 7a) can be readily inkjet-printed on conventional paper and that subsequent UV irradiation of the printed image leads to generation of a blue-colored image associated with formation of a PDA.16 The overall process is depicted in Figure 7b. First, a solution of the amphiphilic DA-1 monomer in aqueous ethanol (100:20 v/v water/ethanol) was loaded into an empty cartridge, which was used to print the monomeric DA-1 image on conventional A4 paper. The printed image is not visible because DA-1 does not absorb visible light. However, a blue-colored image appears on paper upon exposure to UV light (254 nm, 1 mW/cm2). Soft pressing of the skin (e.g., a fingertip) on the blue-colored PDA-coated paper causes an immediate color change from blue to red as well as red fluorescence emission, allowing sweat pore mapping on the skin.

Figure 5. Fluorescence image of the sweat pore pattern of a fingertip printed on a PDA-Cs film. Reproduced with permission from ref 52. Copyright 2014 Nature Publishing Group.

4. INKJET-COMPATIBLE AND TEMPERATURE-DEPENDENT HYDROCHROMIC POLYDIACETYLENE A critical factor that needs to be considered in developing broader applications of hydrochromic materials for various sensor systems (e.g., sweat pore mapping) is their compatibility with the conventional inkjet printing method. The importance of this factor results from the flexible design features of inkjet printers and the ease with which sensor films can be prepared on a large scale in an inexpensive manner.53−55 Thus, inkjet printing of hydrochromic materials on conventional paper

Figure 6. Sweat pore pattern matching. (a) Threshold image of a fluorescent sweat pore pattern of a fingertip deposited on a PDA-Cs film used to obtain coordinates of the sweat pore positions. (b) Sweat pore positions indicated as pink dots are superimposed on a latent form of the same fingerprint developed using ninhydrin. (c, d) Sweat pore pattern matching of the two independent images: the fluorescence microscopy fingerprint image (c) and the latent fingerprint image (d). Blue dots indicate points of matching between the two images. (e, f) Magnified images of the areas marked in (c) and (d). (g) Superimposition of the images in (e) and (f). Reproduced with permission from ref 15. Copyright 2014 Nature Publishing Group. E


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Figure 7. Inkjet-printable imidazolium-modified PDA precursor for sweat pore mapping. (a) Chemical structure of diacetylene (DA) containing the imidazolium group and photographs of PDA derived from DA-1 in response to water. (b) Schematic illustration of the stepwise procedure involving inkjet printing using the DA ink, UV polymerization (254 nm, 1 mW/cm2), and triggering of the color transition from blue to red by water in sweat components. (c) Temperature-dependent color change. The PDA-coated paper does not display hydrochromism when it is in contact with an ice cube, whereas a clear blue-to-red transition occurs immediately when a rubber gloved finger touches the cold water left on the paper. (d) Fluorescence images of sweat pores on fingertips/palm. (e) Magnified image of the boxed area in (d). Reproduced with permission from ref 16. Copyright 2015 Wiley-VCH.

takes place above 20 °C because the monomers in the polymer matrix are dissolved by water at that temperature. The system composed of imidazolium-modified PDA coated on conventional paper was found to be sufficiently sensitive to detect an exceedingly small amount of water, such as that involved in sweat pore mapping. This feature was demonstrated by inkjet printing of DA-1 on an entire piece of paper, which was then UV-irradiated to form a blue-colored paper. From the fluorescence images created when a palm was pressed on the paper (Figure 7d,e), one can see that sweat pore patterns were clearly generated. The results show that the imaging system using imidazolium-modified PDA paper can be employed to produce high-resolution pore maps that are efficient for finger(foot)print analyses. Raman spectroscopy analyses of the imidazolium-containing PDA provided useful information about the mechanism responsible for the water-promoted hydrochromism (Figure 8a,b). The alkyne stretching band in the Raman spectrum shifted from 2079 to 2120 cm−1, and a spectral shift was observed in the alkene stretching region from 1452 to 1515 cm−1. These spectral changes are associated with the

Interestingly, the hydrochromic transition of the imidazolium-modified PDA paper film does not occur optimally at temperatures below 20 °C.16 The temperature dependence of the water-sensing system was demonstrated by inkjet printing of the DA-1 ink on a rectangular region on paper. A blue rectangular image is produced by exposing the paper to UVirradiation. Drawing a thick line on the blue area using cold water does not produce a noticeable color change, whereas a blue-to-red transition occurs when the temperature of the cold wet region is increased by placing a gloved finger on the paper (Figure 7c). The temperature dependence of the hydrochromism is likely a result of the effect of temperature on the solubility of DA monomers. Dissolution of a portion of the unpolymerized monomers captured in a PDA matrix in solvent causes the formation of voids that enables distortion of the PDA main chain present in the blue-phase form. Importantly, we observed that the solubility of imidazolium-modified DA monomers in water sharply increases when the temperature is raised above 20 °C. Therefore, the water-promoted color change of the imidazolium-modified PDA paper preferentially F


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Figure 8. Mechanistic features of the water-promoted colorimetric response of an imidazolium-modified PDA. (a) Raman spectra of the alkyne− alkene stretching region in the PDA backbone before (blue line) and after (red line) water exposure. (b) Alkyl chain stretching region in the Raman spectra of the PDA before (blue line) and after (red line) hydration. (c, d) Schematic of the all-trans (c) and some gauche (d) conformations in the alkyl chain upon hydration. Reproduced with permission from ref 16. Copyright 2015 Wiley-VCH.

Figure 9. Sweat pore mapping using a fluorescein−poly(vinylpyrrolidone) (PVP) composite polymer film. (a) Equilibrium reaction of fluorescein interconverting between the ring-closed nonfluorescent spirolactone form and the ring-opened fluorescent fluorone form. (b) Emission spectra of a fluorescein−PVP film before (black line) and after (red line) hydration. (c) Fluorescence image of a sweat pore pattern of a fingertip deposed on a fluorescein−PVP film. Reproduced with permission from ref 17. Copyright 2015 Royal Society of Chemistry.

conformation of the π-conjugated backbone of the PDA. The C−C stretching region in the Raman spectrum of the blue form of this substance contains key bands that relate to the structure of the aliphatic alkyl chain.56,57 A strong Raman band at 1081 cm−1 and two weak bands at 1105 and 1126 cm−1 are present in the spectrum of the PDA prepared from the imidiazolium-

containing DA-1. The three bands, assigned to alkyl chains in the polymer containing all-trans conformations, disappear and one major band appears at 1068 cm−1 when the film is exposed to water (Figure 8b). This observation suggests that the major all-trans C−C conformations of the alkyl chains in the bluephase PDA (Figure 8c) change to some containing gauche G


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Figure 10. (a) Schematic representation of the formation of supra-CDs from CD-Ps and water-induced disassembly of supra-CDs. (b) Photograph of a fingerprint image printed on supra-CD-coated paper (left, scale bar = 5 mm) and fluorescence microscopy image of the magnified fingerprint of the marked area (right, scale bar = 2 mm). It should be noted that the red-colored-dot image shown in the right panel has been changed from the original green-colored-dot image for better presentation. Reproduced with permission from ref 29. Copyright 2014 Wiley-VCH.

and PVP in dimethylformamide (DMF) was repeatedly spincoated on a glass substrate. A fingerprint was gently applied to the composite polymer film. As shown in Figure 9c, this process led to the formation of fluorone-form-derived green fluorescent microdots corresponding to sweat-secreting pores (i.e., active sweat pores). It should be noted that the use of the hydrophilic polymer PVP enables local adsorption of water into the composite film, where it promotes the ring-opening reaction of fluorescein to produce the ring-opened fluorescing form of the dye. The operation of this mechanism was confirmed using composites of fluorescein with hydrophobic matrix polymers such as polystyrene (PS) and poly(methyl methacrylate) (PMMA), which prevented the penetration of sweat (water) through the polymer matrix. In these cases, fluorescence sweat pore patterns were not generated when fingerprints were deposited on the polymer composite films. The fluorescein− PVP film was also employed for a latent fingerprint analysis. The close correlation seen between the sweat pore fluorescence pattern and the latent fingerprint pattern obtained using the modified ninhydrin staining method clearly demonstrates that the fluorescein-based sweat pore mapping method can be used for personal identification applications. A critical advantage of

conformations in the red-phase PDA (Figure 8d). The results suggest that hydration of the PDA film causes distortion of the backbone of the main chain along with some conformational changes in the aliphatic alkyl chain. These changes lead to less efficient overlap of the p-orbitals in the main-chain backbone, which promotes the blue-to-red color change of the polymer.

5. FLUORESCEIN-BASED SWEAT PORE MAPPING In the most recent phase of our studies in this area, we have devised a simple yet efficient sweat pore mapping method that relies on the use of a fluorescein−hydrophilic polymer composite film.17 Fluorescein is a commercially available small-molecule dye that undergoes water-promoted turn-on fluorescence. As depicted in Figure 9a, the spirolactone form of fluorescein undergoes a ring-opening reaction in the presence of water that leads to the formation of a ring-opened fluorone form. The ring-opened form of the dye is strongly fluorescent, whereas the spirolactone form is nonfluorescent (Figure 9b). On the basis of this unique property, fluorescein was utilized along with the hydrophilic polymer poly(vinylpyrrolidone) (PVP) to create a composite film for human sweat pore mapping. To test the method, a solution of 2 wt % fluorescein H


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Figure 11. Hydrochromic substances and systems that can be potentially applied to mapping of human sweat pores. These compounds and systems have either been reported in the literature (a,1 b,2 c,7 d,9 e,10 f,12 g,14 j3,5,6) or are commercially available (h and i).

the new method is that all of the materials used to fabricate the fluorometric sensor film are commercially available and can be used as received without further purification. This feature is important in terms of mass production of the sensor films using standardized industrial manufacturing processes.

CDs disassemble to form small CD-Ps, which display enhanced luminescence. It was further demonstrated that this waterpromoted enhancement of luminescence on supra-CD-coated paper is sufficiently sensitive to enable detection of a tiny amount of water secreted from active sweat pores and, consequently, to be useful for sweat pore mapping (Figure 10b).

6. CARBON-NANODOT-BASED SWEAT PORE MAPPING By control of their self-assembly behavior, luminescent carbon nanodots (CDs) that possess unique optical properties can be utilized in hydrochromic sensor applications.29 In this approach, the surface of CDs are partially modified by introduction of hydrophobic groups such as those possessing dodecyl alkyl chains (CD-Ps). The amphiphilic nature of the surface produced by using this partial hydrophobic surface modification causes the CD-Ps to form self-assembled suprastructures (supra-CDs) when dissolved in an appropriate solvent (i.e., toluene) (Figure 10a). When individual CD-Ps are well-dispersed in a solvent (i.e., dimethyl sulfoxide, DMSO), the solution displays strong luminescence. In contrast, weak fluorescence emission is observed when CD-Ps form aggregates in a solvent such as toluene. To evaluate their possible use in hydrochromic sensing applications, supra-CDs were coated on paper by dipping conventional filter paper into a toluene solution containing the aggregates for 5 min. The supra-CD-Ps became embedded in the paper matrix, and as a result, the coated paper displays weak luminescence after solvent evaporation. Interestingly, the supraCD-coated paper undergoes an enhancement in its fluorescence emission upon hydration, in which the strong emission is maintained even after evaporation of the water. It was suggested that upon application of water to paper, the supra-

7. CONCLUSIONS AND PERSPECTIVES In this Account, we have described strategies that we have employed to design hydrochromic materials and techniques for human sweat pore mapping. In one approach, incorporation of hygroscopic ions enabled the fabrication of unprecedented water-responsive colorimetric polydiacetylenes. The mechanism of the PDA hydrochromism parallels the general pathway operating in PDA solvatochromism. Thus, water-promoted dissolution of unreacted monomers in the PDA matrix results in the creation of voids, which enable free rotation of PDA chains leading to distortion of the PDA backbone structure. This conformational change causes ineffective p-orbital overlap in the conjugated array and results in a hypsochromic shift of the absorption maximum and a consequent color change. The results of studies with the hydrochromic PDA showed that it can be utilized to map sweat pores on human fingertips. The hydrochromic sweat pore approach was also found to be useful for analysis of latent fingermarks deposited on porous substrates, where dot images rather than conventional ridge patterns are produced. Structural optimization of the PDA headgroup afforded an inkjet-compatible, hydrochromic imidazolium-modified PDA. A significant feature of the new PDA is that it can be fully integrated into conventional inkjet printing on commercial I


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the field of organic and bioorganic chemistry. After postdoctoral studies with Prof. Peter Schultz at the University of CaliforniaBerkeley, he joined KIST (Korea) in 1996. He moved to the Department of Chemical Engineering at Hanyang University in 2000 where he is a full professor. His research focuses on the design and synthesis of organic nanomaterials for sensor applications.

paper. This property provides the new PDA system with several advantageous features, including ease of fabrication, low cost, user-friendliness, and disposability. Finally, an even more simple sweat pore mapping technique was developed using a readily prepared fluorescein−PVP composite film. In addition, hydrochromic carbon nanodots (CDs), recently described by others, have been found to be useful for sweat pore imaging. The four hydrochromic systems described above have their own unique features, such as high sensitivity (PDA-Cs), stability under ambient conditions and producibility on paper using an inkjet printing method (PDA−imidazolium salt), commercially available materials and partial reversibility (fluorescein−PVP), and facile fabrication (carbon nanodots). These new sweat-pore-based fingerprint analysis methods described above have potential promising applications to personal authentication, such as in criminal investigations and license validity checks. The sweat pore patterns are classified as third-level characteristics in forensic science. The development of reliable and affordable sweat pore mapping techniques is highly important in identification of partial or damaged fingerprints, in particular when the first-level (i.e., friction ridge patterns) and second-level (i.e., minutiae features) characteristics are not sufficient or available for identification. Notably, applications to biometric security systems require that colorimetrically reversible hydrochromic materials operate in a reversible manner where the original color of the material is recovered after evaporation of water. As a result, attention should be given in the future to the development of colorimetrically reversible hydrochromic materials for biometric application. In addition, the capability to detect only active sweat pores utilizing the new approaches has potential biomedical applications related to clinical diagnosis of malfunctioning sweat pores. Finally, because the substances and systems shown in Figure 11 display water-promoted color changes, these materials/systems could be potentially employed for sweat pore mapping by introducing proper structural modifications and/or by optimization of the imaging systems.

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ACKNOWLEDGMENTS The studies described in this Account were supported financially by a grant from the National Research Foundation of Korea (NRF) funded by the Government of Korea (MSIP) (2014R1A2A1A01005862) and the Engineering Research Center of Excellence Program of MSIP/NRF (2014R1A5A1009799).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Dong-Hoon Park was born in Cheongju, Korea in 1986. He received his B.S. from Hanyang University in 2012 and is currently pursuing a Ph.D. from Hanyang University under the guidance of Professor JongMan Kim in the Department of Chemical Engineering. His research focuses on the design of polydiacetylene-based solvatochromic sensors. Bum Jun Park was born in 1977 in Incheon, Korea. He received his Ph.D. in Chemical Engineering at the University of Delaware in 2010. After spending two years as a postdoctoral researcher at the University of Pennsylvania, he is currently an assistant professor in the Department of Chemical Engineering at Kyung Hee University. His research interests focus on fabrication of polymeric materials and their interactions/morphology/microstructures. Jong-Man Kim was born in 1964 in Youngcheon City, Kyeongbuk Province, Korea. He received his Ph.D. from the University of Maryland-College Park (1994), advised by Prof. Patrick Mariano in J


Article

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Accounts of Chemical Research from 10,12-Pentacosadiynoic Acid. Chem. Phys. Lett. 2007, 444, 328− 332.

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