Unlocking the full power of electrochemical fingerprinting for on-site sensing applications.

Electrochemical sensing for the semi-quantitative detection of biomarkers, drugs, environmental contaminants, food additives, etc. shows promising results in point-of-care diagnostics and on-site monitoring. More specifically, electrochemical fingerprint (EF)-based sensing strategies are considered an inviting approach for the on-site detection of low molecular weight molecules. The fast growth of electrochemical sensors requires defining the concept of direct electrochemical fingerprinting in sensing. The EF can be defined as the unique electrochemical signal or pattern, mostly recorded by voltammetric techniques, specific for a certain molecule that can be used for its quantitative or semi-quantitative identification in a given analytical context with specified circumstances. The performance of EF-based sensors can be enhanced by considering multiple features of the signal (i.e., oxidation or reduction patterns), in combination with statistical data analysis or sample pretreatments or by including electrode surface modifiers to enrich the EF. In this manuscript, some examples of EF-based sensors, strategies to improve their performances, and open challenges are discussed to unlock the full power of electrochemical fingerprinting for on-site sensing applications. Graphical abstract Electrochemical fingerprint-based sensing strategies can be used for the detection of electroactive analytes, such as antibiotics, phenolic compounds, and drugs of abuse. These strategies show selective and sensitive responses and are easily combined with portable devices.

Extraneous fibre traces brought by river water - A case study.

The fibre traces on a young victim found underwater were mostly single fibre traces besides small amounts of fibre collectives indistinguishable from his parents clothes (mainly wool). Most of those single fibre traces were blue-grey polyester fibres showing tiny differences among each other. They were unexpected according to known population fibre studies. One year after the victim's discovery experiments were conducted to evaluate the possible contamination with fibres from river water. A small amount of extraneous fibres were collected among which blue and grey-black cotton and man-made (mainly polyester) fibres. All man-made fibres were single fibre traces and small fibre collectives were only observed for cotton. These results confirmed the frequent occurrence of blue and grey-black cotton fibres as background, but also highlighted the possible contamination with single blue and grey-black man-made fibres from river water. No wool was found, strengthening the significance of the wool fibre collectives present on the victim.

10 years of 1:1 taping in Belgium - A selection of murder cases involving fibre examination.

Since 2002, the 1:1 tape lifting technique on murder victims has been introduced in Belgium. Thirty-six murder cases for which a fibre examination was carried out in our laboratory between 2002 and 2012 are reviewed. These cases all involved the collection of microtraces on 39 victims using a 1:1 taping or similar technique, called semi 1:1 taping. The degree of "positive outcomes" for investigative and comparative fibre cases is high. Especially in cases where the modus operandi of suspects had to be verified, the examination of the 1:1 tape lifting led to very useful fibre results. The findings from some of these fibre cases are used to illustrate the advantages, drawbacks and points of interest of the two collection techniques. These are also compared with the zonal taping technique, another microtrace collection technique applied on victims at the crime scene.

Use of potentiometric sensors to study (bio)molecular interactions.

Potentiometric sensors were used to study molecular interactions in liquid environments with sensorgram methodology. This is demonstrated with a lipophilic rubber-based and a collagen-based hydrogel sensor coating. The investigated molecules were promazine and tartaric acid, respectively. The sensors were placed in a hydrodynamic wall-jet system for the recording of sensorgrams. Millivolt sensor responses were first converted to a signal, expressing the concentration of adsorbed organic ions. Using a linearization method, a pseudo-first order-kinetic model of adsorption was shown to fit the experimental results perfectly. K(assoc), k(on), and k(off) values were calculated. The technique can be used over 4 decades of concentration, and it is very sensitive to low-MW compounds as well as to multiply charged large biomolecules. This study is the first to demonstrate the application of potentiometric sensors as an alternative and complement to surface plasmon resonance methods.

Dichroism measurements in forensic fibre examination. Part 4-dyed acrylic and acetate fibres.

A number of dyed acrylic and acetate fibre samples were examined with plane polarized light on their dichroic behavior by optical light microscopy (OLM) and microspectrophotometry with plane polarized light (MSP-PPL). It was found that most of these low birefringent fibres possess weak dichroic effects that are very hard to observe with microscopy. However, using MSP-PPL, the linear dichroism could be measured. A comparison between the dichroic effects found for the same disperse dyes on triacetate (TrAc), diacetate (Ac), polyester (PES) and polyamide (PA) shows that the linear dichroism follows the order: PA>PES >>TrAc, Ac.

Dichroism measurements in forensic fibre examination: part 5-pigmented fibres.

A number of pigmented fibre samples were examined with plane polarized light on their dichroic behaviour by optical light microscopy (OLM) and microspectrophotometry with plane polarized light (MSP-PPL). It was found that about half of the samples show a strong dichroic effect and another 20% have a weak dichroism. Both regular (80%) and inversed dichroic effects (20%) occur. The dichroic characteristics of pigmented fibres can be compared to these of sheet polarizers. It is suggested that the dichroic behaviour of pigmented fibres depends strongly on the crystal structure (shape of the pigment grains) and the draw ratio (orientation of the polymer chains).

The use of linear dichroism in forensic fibre examinations--part 6. Validation and practical aspects of MSP-PPL.

This paper summarizes the results of previous work on the microscopic observation of linear dichroism found in dyed fibres (polyesters, polyamides, wool, silk, cotton, viscose, acrylics and acetates) and in pigmented fibres as well as the measurements on these fibre classes using microspectrophotometry with plane polarized light (MSP-PPL). The validation of this method is discussed and a practical tool is proposed for comparing fibre traces with control fibres. The limitations and strengths of this method are also revised.

Dichroism measurements in forensic fibre examination. Part 3 - dyed cotton and viscose fibres.

A number of dyed cellulosic fibres were examined with plane polarized light on their dichroic behavior by microscopy and microspectrophotometry (MSP-PPL). Significant dichroic effects (mostly hypochromic effects and hypsochromic bands shifts) were reported. The effect is related to the chemical structure: some dye structures always possess dichroism (azo, stilbene, thiazole and oxazine), some dyes demonstrate sometimes dichroic effects (anthraquinoid, indigoid) while other structures never demonstrate dichroic effects (sulphur, diphenylmethanes, triarylmethanes, phthalocyanines). In some cases a different dichroic behavior was found for the same dyes applied on cotton and on viscose.

Dichroism measurements in forensic fibre examination Part 1--Dyed polyester fibres.

One hundred and twenty dyed polyester samples were examined with plane polarized light on their dichroic behaviour by optical light microscopy (OLM) and microspectrophotometry in the visible range (MSP Vis). It was found that most of these disperse dyed polyester fibres possess a strong dichroism, which fall into two broad categories. Either a decrease of intensity (hypochromic effect) or a change of hue (hypsochromic or bathochromic shift of absorption bands) is noted. These dichroic effects are related to the orientation of the dye structure with respect to the polymer chains.

Dichroism measurements in forensic fibre examination. Part 2 - dyed polyamide, wool and silk fibres.

A number of dyed polyamide, wool and silk samples were examined with plane polarized light on their dichroic behavior by optical light microscopy (OLM) and microspectrophotometry with plane polarized light (MSP-PPL). It was found that most of these acid dyed peptidic fibres possess dichroism, but these are weaker than the effects previously described for polyester fibres. The small effects may be not observed, especially for wool, but these can be measured using MSP-PPL. In the three peptidic fibre classes, for the first time, a so called "inverse dichroism" is observed which appears in the absorption spectra as a hyperchromic effect.

Challenges for cocaine detection in smuggling samples.

Screening of seized cocaine powders is routinely performed by means of colour tests. An alternative fast screening technique is Mid-InfraRed (MIR) spectroscopy. In the context of smuggling cases, however, drugs are often processed to circumvent detection. In this study, the current screening techniques (cocaine colour test and MIR spectroscopy using libraries and chemometrics) were applied to five smuggling cases. For each case, all samples were first screened with a cocaine colour test and MIR analysis, followed by confirmation analyses with GC-MS and GC-FID to identify and quantify cocaine and cutting agents. Finally, Scanning Electron Microscopy-Energy Dispersive X-ray spectroscopy (SEM-EDX) analyses were performed for additional characterization. All smuggling samples tested negative, both on-site as in the laboratory, for cocaine with the cocaine colour test. Four smuggling cases consisted of coloured samples. Consequently the colour test result was influenced because discolouration of the test showed almost the same colour as the colour of the powders (brown, green, red or black). In contrast, the (coloured) powders could be measured with MIR, but the MIR spectra showed no hit for cocaine using a reference library search. Moreover, cocaine was not detected in four out of the five cases after application of a chemometric classification model. GC-MS analysis, the golden standard for identification, resulted in a positive identification of cocaine in all cases. These samples contained cocaine ranging between 0.8w% and 35w%. Taking into account the results of the screening, the chromatographic and the SEM-EDX analyses, it could be presumed that cocaine was masked. False negative screening results were caused by chemically modified cocaine and/or cocaine mixed with coloured powders. In additional experiments, a sample extraction step prior to the screening techniques was performed. Two sample preparation methods (acetone and ethyl acetate) were tested and resulted in a positive screening of cocaine with the colour test and/or MIR spectroscopy. It can be concluded that the outcome of screening techniques such as colour tests and MIR spectroscopy is only presumptive and should always be confirmed.

Evaluation of the shedding potential of textile materials.

A practical method is proposed to assess the shedding potential of textile materials. The evaluation of the sheddability is based on the use of adhesive tapes and on comparison, using three different sheddability scales for cotton, wool and fine man-made fibres.

Evaluation of a calibration transfer between a bench top and portable Mid-InfraRed spectrometer for cocaine classification and quantification.

A portable Fourier Transform Mid-InfraRed (FT-MIR) spectrometer using Attenuated Total Reflectance (ATR) sampling is used for daily routine screening of seized powders. Earlier, ATR-FT-MIR combined with Support Vector Machines (SVM) algorithms resulted in a significant improvement of the screening method to a reliable and straightforward classification and quantification tool for both cocaine and levamisole. However, can this tool be transferred to new (hand-held) devices, without loss of the extensive data set? The objective of this study was to perform a calibration transfer between a newly purchased bench top (BT) spectrometer and a portable (P) spectrometer with existing calibration models. Both instruments are from the same brand and have identical characteristics and acquisition parameters (FT instrument, resolution of 4 cm-1 and wavenumber range 4000 to 500 cm-1). The original SVM classification model (n = 515) and SVM quantification model (n = 378) were considered for the transfer trial. Three calibration transfer strategies were assessed: 1) adjustment of slope and bias; 2) correction of spectra from the new instrument BT to P using Piecewise Direct Standardization (PDS) and 3) building a new mixed instrument model with spectra of both instruments. For each approach, additional cocaine powders were measured (n = 682) and the results were compared with GC-MS and GC-FID. The development of a mixed instrument model was the most successful in terms of performance. The future strategy of a mixed model allows applying the models, developed in the laboratory, to portable instruments that are used on-site, and vice versa. The approach offers opportunities to exchange data within a network of forensic laboratories using other FT-MIR spectrometers.

Application of Raman spectroscopy to forensic fibre cases.

Five forensic fibre cases in which Raman spectroscopy proved to be a good complementary method for microspectrophotometry (MSP) are described. Absorption spectra in the visible range are indeed sometimes characteristic ofa certain dye but this one can be subsequently identified unambiguously by Raman spectroscopy using a spectral library. In other cases the comparison of Raman spectra of reference fibres and suspect fibres led to an improvement of the discrimination power. The Raman measurements have been performed directly on mounted fibres and the spectra showed only little interference from the mounting resin and glass. Raman spectroscopy is therefore a powerful method that can be applied in routine fibre analysis following optical microscopy and MSP measurements.

Practical tool for sampling and fast analysis of large cocaine seizures.

Large quantities of illicit drugs are frequently seized by law enforcement. In such cases, a representative number of samples needs to be quickly examined prior to destruction. No procedure has yet been set up which rapidly provides information regarding the homogeneity of the samples, the presence of controlled substances, and the degree of purity. This study establishes a protocol for fast analysis of cocaine and its most common cutting agent, levamisole, in large seizures. The protocol is based on a hypergeometric sampling approach combined with Fourier-transform infrared (FTIR) spectrometry and support vector machines (SVM) algorithms as analysis methods. To demonstrate the practical use of this approach, 5 large cocaine seizures (consisting between 45 and 85 units) were analysed simultaneously with gas chromatography-mass spectrometry (GC-MS), gas chromatography-flame ionisation detector (GC-FID), and a portable FTIR spectrometer using attenuated total reflectance (ATR) sampling combined with SVM models. According to the hypergeometric sampling plan of the guidelines of the Drugs Working Group (DWG) of the European Network of Forensic Science Institutes (ENFSI), the required number of subsamples ranged between 19 and 23. Considering the identification analyses, the SVM models detected cocaine and levamisole in all subsamples of Cases 1 to 5 (100% correct classification), which was confirmed by GC-MS analysis. Considering the quantification analyses, the SVM models were able to estimate the cocaine and levamisole content in each subsample, compared to GC-FID data. The developed strategy is easy, cost effective, and provides immediate information about both the presence and concentration of cocaine and levamisole. By using this new strategy, the number of confirmation analyses with laborious and expensive chromatographic techniques could be significantly reduced.

A comparison of efficiency of manual and automatic fibres search with the Maxcan fibre finder.

The aim of this work was to study the efficiency of automatic fibre searching with the Maxcan fibre finder (Cox Analytical Systems, Sweden) in comparison to manual searching. The influence of some parameters (color, thickness, background noise) on the results of a fibre search was considered. Eighteen experimental tapes with different target fibres and different background noises were prepared in the laboratory. Searching of fibres was performed manually and with the Maxcan fibre finder by different operators from four European laboratories. Two laboratories have the Maxcan fibre finder system and the two instruments were used and compared in this study. The results show that searching with the Maxcan is generally as efficient as manual searching, except for very pale or very dark fibres. Note that the tapes used for these experiments were prepared in laboratory, and are not completely representative of the tape that could be obtained in real cases. To generalize the results obtained, further research on real case samples would be necessary.

Morphological details in bloodstain particles.

During the commission of crimes blood can be transferred to the clothing of the offender or on other crime related objects. Bloodstain particles are sub-millimetre sized flakes that are lost from dried bloodstains. The nature of these red particles is easily confirmed using spectroscopic methods. In casework, bloodstain particles showing highly detailed morphological features were observed. These provided a rationale for a series of experiments described in this work. It was found that the "largest" particles are shed from blood deposited on polyester and polyamide woven fabrics. No particles are lost from the stains made on absorbent fabrics and from those made on knitted fabrics. The morphological features observed in bloodstain particles can provide important information on the substrates from which they were lost.

Discrimination of reactively-dyed cotton fibres with thin layer chromatography and UV microspectrophotometry.

Reactively-dyed black, navy blue and medium red cotton samples showing metamerism under fluorescent tube illumination were examined. Optical microscopy (bright field, polarization and fluorescence microscopy) was used, followed by microspectrometry in the visible range (MSP Vis), to differentiate the samples in each block of colours. Additionally, the non-discriminated samples were subjected both to microspectrophotometry in the UV-range (MSP UV) and to enzymatic digestion followed by high performance thin layer chromatography (HPTLC) on the digests. While it was found that both methods may potentially result in higher discrimination, preparation of reactively-dyed cotton for HPTLC was found to be a very tedious and time-consuming step and HPTLC only led to a better discrimination than MSP UV for the red cotton samples. The results suggest that in order to increase the discrimination for reactively-dyed cotton fibres, measurement of the UV absorption spectrum (MSP UV) is preferred over HPTLC.

Fibre persistence on immersed garment-Influence of knitted recipient fabrics.

The persistence of fibre traces is a critical factor in the evaluation of fibre findings in forensic casework. Many persistence studies have examined the loss of fibre traces on living subjects or on bodies exposed to open-air conditions but none of them have ever considered bodies in underwater conditions. This first part of a larger study focuses on the persistence of transferred fibres on various recipient garments during an immersion/emersion process as the common step to all casework involving underwater conditions. A smooth polyester garment leads to weak persistence values around 20%, while higher persistence values, ranging between 80% and 90%, are obtained for more structured and textured garments (cotton T-shirts, fleece and acrylic pullovers). More textured garments are thus less affected by the immersion/emersion process and fibre findings are closer to their initial amount and distribution before immersion. The amount of protruding fibres and the density of the rough fibrous network at the surface of the recipient garment are both key factors that increase the persistence value. Therefore, these factors should be taken into account when evaluating fibre evidence recovered on immersed bodies.

Potentiometric sensors doped with biomolecules as a new approach to small molecule/biomolecule binding kinetics analysis.

The most successful binding kinetics analysis systems at this moment include surface plasmon resonance (SPR), quartz microcrystal balance (QMB) and surface acoustic wave (SAW). Although these are powerful methods, they generally are complex, expensive and require the use of monolayers. Here, we report on potentiometric sensors as an inexpensive and simple alternative to do binding kinetics analysis between small molecules in solution and biomolecules (covalently) attached in a biopolymer sensor coating layer. As an example, dopamine and an anti-dopamine aptamer were used as the small molecule and the biomolecule respectively. Binding between both follows a Langmuir adsorption type model and creates a surface potential. The system operates in Flow Injection Analysis mode (FIA). Besides being an interesting new binding kinetics tool, the approach allows systematic design of potentiometric biosensors (in the present study a dopamine sensor), and gives new insights into the functioning of ion-selective electrodes (ISE's).