Electrochromic enhancement of latent fingerprints by poly(3,4-ethylenedioxythiophene)

Spatially selective electrodeposition of poly-3,4-ethylenedioxythiophene (PEDOT) thin films on metallic surfaces is shown to be an effective means of visualizing latent fingerprints. The technique exploits the fingerprint deposit as an insulating mask, such that electrochemical processes (here, polymer deposition) may only take place on deposit-free areas of the surface between the ridges of the fingerprint deposit; the end result is a negative image of the fingermark. Use of a surfactant (sodium dodecylsulphate, SDS) to solubilise the EDOT monomer allows the use of an aqueous electrolyte. Electrochemical (coulometric) data provide a total assay of deposited material, yielding spatially averaged film thicknesses, which are commensurate with substantive filling of the trenches between fingerprint deposit ridges, but not overfilling to the extent that the ridge detail is covered. This is confirmed by optical microscopy and AFM images, which show continuous polymer deposition within the trenches and good definition at the ridge edges. Stainless steel substrates treated in this manner and transferred to background electrolyte (aqueous sulphuric acid) showed enhanced fingerprints when the contrast between the polymer background and fingerprint deposit was optimised using the electrochromic properties of the PEDOT films. The facility of the method to reveal fingerprints of various ages and subjected to plausible environmental histories was demonstrated. Comparison of this enhancement methodology with commonly used fingerprint enhancement methods (dusting with powder, application of wet powder suspensions and cyanoacrylate fuming) showed promising performance in selected scenarios of practical interest

Novel hybrid based on a poly[Ni(salen)] film and WO3 nanoparticles with electrochromic properties

The strategy of combining electroactive polymers and inorganic nanomaterials has been widely explored in recent years in order to improve some of their properties, namely electrocatalysis and electrochromism. This report focuses on a new composite prepared through the electropolymerization of the transition metal complex [Ni(3-Mesalen)], designated as [1] in the presence of WO3 nanoparticles (NPs) and its electrochromic (EC) performance. The WO3 NPs were prepared using tungsten metal powder; their characterization indicated quasi-spherical morphology, high crystallinity and particle sizes in the range 30–40 nm. The nanocomposite WO3@poly[1] films displayed similar electrochemical responses to those of pristine poly[1] films in LiClO4/CH3CN, but higher electroactive surface coverages, an advantage of NPs incorporation in the nanocomposite. The presence of the WO3 NPs in the poly[1] matrix was assessed by X-ray photoelectron spectroscopy and scanning electronic microscopy. The nanocomposite presented similar electronic spectra to those of poly[1], indicating that the electronic structure of the pristine film is maintained in the nanocomposite, but exhibited lower ε-values for bands associated with charge transfer transitions for high oxidised states, revealing an enhanced stability towards ligand over-oxidation. The WO3@poly[1] nanocomposite showed more favourable EC properties in LiClO4/CH3CN than the pristine film. For typical coverages (Γ = 0.06-0.10 μmol cm−2) the composite showed lower switching times (τ = 1.3 − 3.6 s), higher optical contrast (ΔT ≈ 31%, an improvement of ca. 40%) and better colouration efficiencies (in the range η = 104 − 115 cm2C−1, improvement of ca. 13 − 22%)

Multicolour Electrochromic Film Based on a TiO2@poly[Ni(salen)] Nanocomposite with Excellent Electrochemical Stability

We report the electrochromic properties of a polymeric nanocomposite prepared by potentiodynamic deposition of transition-metal complex [Ni(3-Mesalen)], designated as [1], in the presence of TiO2 nanoparticles (NPs) with an average size of 9.7 ± 1.1 nm. Entrapment of TiO2 NPs in the poly[1] matrix was confirmed by several techniques. The nanocomposite TiO2@poly[1] films showed similar electrochemical responses to the original (nanoparticle-free) poly[1] films, but with higher electroactive surface coverages (Γ), showing the advantage of the nanocomposite preparation. The results indicated that the electronic structure of poly[1] was retained in the nanocomposite; nonetheless, a lower ε value was obtained for the charge-transfer band of the former, revealing superior stability of the nanocomposite for ligand high oxidation states. The TiO2@poly[1] nanocomposite showed interesting color changes, from yellow (reduced state) to green and russet (oxidized states), with enhanced electrochemical stability, demonstrated by a charge loss of only 7.3% over ca. 10 000 redox cycles surpassing the original polymer film stability: the loss of electroactivity is a factor of ca. 2 less than for pristine poly[1]. Furthermore, an enhancement of 16.7% in the optical modulation (ΔOD = 0.48) was also observed for the nanocomposite, confirming the benefit of TiO2 incorporation into the EC properties of the original polymer film

Multicolour Electrochromic Film Based on a TiO₂@poly[Ni(<i>salen</i>)] Nanocomposite with Excellent Electrochemical Stability

We report the electrochromic properties of a polymeric nanocomposite prepared by potentiodynamic deposition of transition-metal complex [Ni­(3-Mesalen)], designated as [1], in the presence of TiO₂ nanoparticles (NPs) with an average size of 9.7 ± 1.1 nm. Entrapment of TiO₂ NPs in the poly[1] matrix was confirmed by several techniques. The nanocomposite TiO₂@poly­[1] films showed similar electrochemical responses to the original (nanoparticle-free) poly[1] films, but with higher electroactive surface coverages (Γ), showing the advantage of the nanocomposite preparation. The results indicated that the electronic structure of poly[1] was retained in the nanocomposite; nonetheless, a lower ε value was obtained for the charge-transfer band of the former, revealing superior stability of the nanocomposite for ligand high oxidation states. The TiO₂@poly­[1] nanocomposite showed interesting color changes, from yellow (reduced state) to green and russet (oxidized states), with enhanced electrochemical stability, demonstrated by a charge loss of only 7.3% over ca. 10 000 redox cycles surpassing the original polymer film stability: the loss of electroactivity is a factor of ca. 2 less than for pristine poly[1]. Furthermore, an enhancement of 16.7% in the optical modulation (ΔOD = 0.48) was also observed for the nanocomposite, confirming the benefit of TiO₂ incorporation into the EC properties of the original polymer film

Fundamental aspects of electrochemically controlled wetting of nanoscale composite materials

Electroactive films based on conducting polymers have numerous potential applications, but practical devices frequently require a combination of properties not met by a single component. This has prompted an extension to composite materials, notably those in which particulates are immobilised within a polymer film. Irrespective of the polymer and the intended application, film wetting is important: by various means, it facilitates transport processes - of electronic charge, charge-balancing counter ions ("dopant") and analyte/reactant molecules - and motion of polymer segments. While film solvent content and transfer have been widely studied for pristine polymer films exposed to molecular solvents, extension to non-conventional solvents (such as ionic liquids) or to composite films has been given much less attention. Here we consider such cases based on polyaniline films. We explore two factors, the nature of the electrolyte (solvent and film-permeating ions) and the effect of introducing particulate species into the film. In the first instance, we compare film behaviours when exposed to a conventional protic solvent (water) with an aprotic ionic liquid (Ethaline) and the intermediate case of a protic ionic liquid (Oxaline). Secondly, we explore the effect of inclusion of physically diverse particulates: multi-walled carbon nanotubes, graphite or molybdenum dioxide. We use electrochemistry to control and monitor the film redox state and change therein, and acoustic wave measurements to diagnose rheologically vs. gravimetrically determined response. The outcomes provide insights of relevance to future practical applications, including charge/discharge rates and cycle life for energy storage devices, "salt" transfer in water purification technologies, and the extent of film "memory" of previous environments when sequentially exposed to different media

N-doped few-layered graphene-polyNi complex nanocomposite with excellent electrochromic properties

A new nanocomposite was obtained through the incorporation of N-doped few-layered graphene (N-FLG) into films of the electroactive polymer poly[Ni(3-Mesalen)] (poly[1]). The nanocomposite, N-FLG@poly[1], prepared by in situ electropolymerization, showed similar electrochemical responses to pristine poly[1], but with more well-defined redox peaks and higher current intensities, in compliance with larger electroactive surface coverage. N-FLG incorporation did not affect the electronic structure of poly[1], but decreased in 12% the molar extinction coefficient of the charge transfer band between metal and oxidized ligand, which is a promising advantage since this band is related to polymer degradation. The N-FLG@poly[1] showed multi-electrochromic behaviour (yellow in reduced state and green/russet in oxidized states) and revealed excellent improvement in electrochromic performance compared to original poly[1], specifically an increase of 71% in electrochemical stability (loss of 2.7% in charge after 10 000 switching cycles). Furthermore, nanocomposite formation decreased the switching time for oxidation (reduction) τ = 9 s (11 s) and improved the optical contrast (ΔT = 35.9%; increase of 38%) and colouration efficiency (η = 108.9 cm2 C−1; increase of 12%), for a representative film of coverage Γ = 296 nmol cm−2. The excellent electrochromic performance improvements are attributed to the alternative conducting pathways and to morphological modifications induced by N-FLG

Effect of electrochemical control function on the internal structure and composition of electrodeposited polypyrrole films: A neutron reflectometry study

Electrodeposited conducting polymer films derived from aromatic monomers are known to possess properties that depend significantly on the deposition protocol, particularly the electrochemical control function employed. This study explores the underlying reasons for this common observation for the specific case of polypyrrole films deposited from aqueous media onto gold electrodes under potentiostatic, potentiodynamic and galvanostatic control. Although the control functions impose different conditions, the control parameters (potential, potential range and scan rate, and current) were selected so as generate films at comparable rates; this avoids inappropriate attribution of structural and compositional variations to different thickness regimes, irrespective of how they were generated. In each case, film deposition was periodically interrupted and the film characterised by specular neutron reflectivity measurements. By using d4-pyrrole monomer in H2O solvent, the isotopic selectivity of neutron reflectivity was used to extract polymer and solvent concentration profiles as a function of distance from the electrode/film interface. Spatial integration of these profiles was used to quantify total film solvent populations; these are expressed as solvent volume fractions. Films grown under the three different control regimes have measurably distinct solvent volume fraction profiles and there is evolution of these profiles with increasing thickness. Ultimately, for the conditions employed, the order of increasing porosity (i.e. solvent content) by control function was potentiostatic < potentiodynamic < galvanostatic. At the end of the deposition process, the films were transferred to monomer-free electrolyte and redox cycled. This resulted in an overall increase in film solvation, but little difference in solvation with redox state (doping level). We conclude that film structure and associated solvation level do retain some memory of deposition protocol, but also respond to the medium of exposure

Nanoscale control of interfacial processes for latent fingerprint enhancement

Latent fingerprints on metal surfaces may be visualized by exploiting the insulating characteristics of the fingerprint deposit as a “mask” to direct electrodeposition of an electroactive polymer to the bare metal between the fingerprint ridges. This approach is complementary to most latent fingerprint enhancement methods, which involve physical or chemical interaction with the fingerprint residue. It has the advantages of sensitivity (a nanoscale residue can block electron transfer) and, using a suitable polymer, optimization of visual contrast. This study extends the concept in two significant respects. First, it explores the feasibility of combining observation based on optical absorption with observation based on fluorescence. Second, it extends the methodology to materials (here, polypyrrole) that may undergo post-deposition substitution chemistry, here binding of a fluorophore whose size and geometry preclude direct polymerization of the functionalised monomer. The scenario involves a lateral spatial image (the whole fingerprint, first level detail) at the centimetre scale, with identification features (minutiae, second level detail) at the 100–200 μm scale and finer features (third level detail) at the 10–50 μm scale. However, the strategy used requires vertical spatial control of the (electro)chemistry at the 10–100 nm scale. We show that this can be accomplished by polymerization of pyrrole functionalised with a good leaving group, ester-bound FMOC, which can be hydrolysed and eluted from the deposited polymer to generate solvent “voids”. Overall the “void” volume and the resulting effect on polymer dynamics facilitate entry and amide bonding of Dylight 649 NHS ester, a large fluorophore. FTIR spectra demonstrate the spatially integrated compositional changes. Both the hydrolysis and fluorophore functionalization were followed using neutron reflectivity to determine vertical spatial composition variations, which control image development in the lateral direction