Mats de Jong ac, Nick Sleegers a, Jayoung Kim b, Filip Van Durme c, Nele Samyn c, Joseph Wang b and also Karolien De Wael *a aAXES Research Group, Chemistry Department, Groenenborgerlaan 171, 2020 Antwerp, Belgium. E-mail: karolien.dewael
uantwerpen.be bDepartment of Nanodesign, College California San Diego, CA 92093, USA cNational Institute for Criminalistics and Criminology (NICC), Vilvoordsesteenweg 100, 1120 Brussels, Belgium


Abstract

We report on a wearable fingerguideline sensor for on-the-spot identification of cocaine and its cutting agents in street samples. Traditionally, on-site screening is performed by indicates of colour tests which are hard to translate and also absence selectivity. By presenting the unique voltammetric response of cocaine, cutting agents, binary mixtures of cocaine and street samples in solution and powder street samples, we were able to elucidate the electrochemical fingerprint of all these compounds. The brand-new electrochemical idea holds significant promise as an on-site screening technique.

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Introduction

Cocaine is just one of the many plentiful drugs of abuse global, entering different countries mostly through airports and also harbours, both for regional intake and for distribution.1 This alkaloid drug is extremely addictive and harmful for people"s health and wellness.2,3 In enhancement to the desired effects for individuals such as an intense euphoric feeling, undesirable additional effects like an increased blood press, heartbeat price and respiration price are skilled.4,5 After irreversible use, the addict becomes tolerant for the desired effects and will experience a state of lethargy, significant depression and also excessive exhaustion as soon as no longer making use of cocaine.4

Customs solutions at airports and harbours are incredibly keen to monitor passing cargo, luggage and also world for the existence of cocaine. On-site screening by suggests of colour tests based on cobalt thiocyanate is frequently percreated. However before, these tests are hard to interpret and also not constantly reliable because of a poor selectivity.6,7 Usually the colour tests are followed by a confirmation analysis by means of GC-MS (qualitative), GC-FID (quantitative) and HPLC for an unambiguous identification of cocaine and its cutting agents. This confirmation supplies a range of industrious and also complex techniques and/or bulky and also expensive tools that have to be avoided for regimen screenings.8 An alternative screening can be performed by FT-IR and also Raman spectroscopy, using expensive instrumentation offering spectra that are difficult to translate without specialized expertise.9 To overcome these constraints, the present examine intends at a quick, cheap, straightforward, and also selective screening test for on-site detection of cocaine.

Knowing that cocaine is a redox active molecule,10 an electrochemical screening strategy by recording voltammograms allows custom-mades to easily display unwell-known powders in cargo and also baggage on-site, facilitating high throughput and low expense of detection. Even even more, most cutting agents can be sensed along with cocaine bereason of their redox task.11–18 These cutting agents are included to cocaine by the dealers, mainly in order to maximise profit. Typical substances are phenacetin, caffeine, levamisole, lidocaine, paracetamol, procaine, benzocaine, diltiazem, hydroxyzine, boric and also benzoic acid, and several sugars choose D-sorbitol, myo-inositol, maltose and also starch. These compounds all have a white colour choose cocaine and also some of them, for instance lidocaine, paracetamol and also phenacetin, have actually analgesic and anesthetic results the users will relate to cocaine.17,18 Voltammograms contain much more indevelopment for identification of street samples containing both cocaine and cutting agents compared to the conventional colour tests that depend exclusively on the existence of cocaine. Here we explain the usefulness of electrochemical methods to meacertain at the same time cocaine and its cutting agents in a single voltammetric run using square-wave voltammeattempt (SWV). In order to achieve the complete and also distinctive electrochemical fingerprint of street samples, all cutting agents and cocaine have been electrochemically screened separately in solution and in a binary mixture. Secondly, powder samples were analysed by using the electrochemical approach, permitting a quick and straightforward on-site detection, without an extensive sample preparation procedure. The sensing tool being included into a glove/fingerreminder, permits such on-site detection.19 The measuring protocol is presented in Fig. 1. Part (A) represents the finger exhibiting the 3 electrode surchallenge screen-printed onto a functional finger cot (bottom left inset), and gel immobilized upon a similar substrate (bottom ideal inset). Part (B) and (C) show, respectively, the ‘swipe’ approach of sampling to collect the taracquire powder directly onto the electrode and the completion of the electrochemical cell by joining the index finger via electrodes to the thumb coated through the solid hydrogel electrolyte. The electrochemical fingerprint is taped in less than a minute by making use of a quick square-wave voltammetric technique, offering indevelopment on both the presence of cocaine and its cutting agents.


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Fig. 1 Schematic of the measuring procedure for suspicious powder samples at a wearable fingerreminder device. (A) The fingertip exhibiting the surchallenge of screen-printed electrode onto a functional nitrile finger cot (bottom left inset), and a conductive gel immobilized upon the thumb (bottom ideal inset); (B) swiping technique of sampling to collect the target powder directly onto the electrode; (C) completion of the electrochemical cell by joining the index finger via electrodes to the thumb coated via the conductive gel electrolyte.

We mean that this new electrochemical fingerprint on a wearable glove platcreate will significantly boost the on-site screening of cocaine samples. The capacity to promptly geneprice this signature via a single sdeserve to approach using compact, cheap and easy-to-usage electrochemical gadgets need to be incredibly useful for on-site detection, perdeveloped by customs services. For the initially time, the visibility of cutting agents in street samples is elucidated in an on-site test.

Results and discussion

Voltammetry of pure compounds in solution

A first and vital action is the screening and also assessment of the redox behaviour of cocaine and also its cutting agents in solution to unravel the electrochemical fingerprint of these compounds. Square-wave voltammograms of 1 mM cocaine or cutting agent in a 0.1 M KCl + 0.020 M KH2PO4 solution at a bare carbon screen-printed electrode are shown in Fig. 2, corrected for the background present by using the relocating average principle, integrated in the NOVA 1.11 software. The 1 mM cocaine solution exhibits a weak oxidation procedure at 1.04 V which can be added to the oxidation of the tertiary amine group current in the hexagon framework of cocaine as is presented in Scheme 1.10 Most cocaine samples are cut with different substances; identification of these cutting agents could be of interemainder from a toxicological point of watch. The parameters of the supplied square-wave voltammetry procedure were optimized prior the analysis and are explained in the experimental component.
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Fig. 2 Baseline-corrected square-wave voltammetric responses (vs. Ag/AgCl) of 1 mM options of cocaine and cutting agents at bare carbon screen-published electrodes in 0.1 M KCl + 0.020 M KH2PO4 buffer (pH 7). The dotted line represents the characteristic oxidation potential of cocaine at 1.04 V. The y-axis has the very same scale in eincredibly voltammogram.

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Scheme 1 Oxidation process of cocaine at a carbon screen-published electrode at potential 1.04 V. The second reactivity occurs instantly after the formation of the intermediate radical, leading to just one visible height in a square-wave voltammogram.

Cutting agents benzoic acid and also boric acid display no redox activity over the studied potential array, which corresponds via the literary works, only showing task at potentials below −1 V for these compounds.14,15 The sugars D-sorbitol, maltose, starch and myo-inositol likewise present no redox activity in the stupassed away potential variety. Fig. 2 illustprices that the cocaine signal at 1.04 V is quite isolated from the various other characteristic signals of the cutting agents. The closest even more positive process is the height of levamisingle at 1.27 V and also the closest more negative is the top of lidocaine at 0.90 V and its analogues procaine and also benzocaine, 0.87 V and 0.84 V, respectively. All these oxidation processes happen at tertiary (levamisole and also lidocaine) and primary (benzocaine and procaine) amine functional teams.11,12,17,20 Acomponent from these height potential worths, the number of characteristic peaks as well as the onset potentials give valuable indevelopment on determining their fingerprint. Phenacetin, for instance, display screens three definitive redox processes, so even if there is an overlap via the main oxidation optimal, the existing cutting agent have the right to still be determined by its second or tertiary signal. The primary signal at 0.77 V results from the irreversible oxidation of phenacetin to N-acetyl-p-benzoquinone imine (NAPQI). The smaller sized additional peaks at 0.06 and 0.35 V are a result of the oxidation of 4-aminophenol and NAPQI, respectively.21 These two second peaks are likewise existing for paracetamol and are caused by the very same compounds.18 Hydroxyzine and diltiazem both present an oxidation optimal at 0.87 V, brought about by the oxidation of a complimentary –OH group and also a tertiary amine, respectively.16,22 Caffeine has actually the highest possible oxidation potential at 1.31 V, resulting from the oxidation of the compound to a 4,5-diol analogue.13

This substantial screening and affluent information content leads to a distinctive electrochemical fingerprint of cocaine and also its cutting agents that can serve as an effective recommendation when the presence of cocaine in an unknown sample demands to be confirmed or ruled out.

The applied conditioning step at 1.5 V (as is explained in the experimental section) was perdeveloped to extract maximal indevelopment from our substances. For instance, phenacetin would certainly only show a solitary height at 0.77 V instead of three peaks if no conditioning action was perdeveloped. Due to the conditioning step, redox procedures concerned NAPQI and 4-aminophenol take place, enriching the fingerprint of the samples.

The limit of detection (LOD) for pure cocaine on the non-modified screen-published electrode (SPE) surface in solution was determined at 2 μM. This synchronizes to a amount of 34 ng on the electrode surconfront. The LOD was established based upon the conventional deviation of the intercept and also the average slope of the direct calibration curves that were acquired.

This LOD is significantly reduced compared to the colour screening tests (13.8 μM) and the accredited GC-MS method provided at the National Institute of Criminalistics and also Criminology (NICC) of Belgium (18.4 μM).

Voltammeattempt of binary mixtures in solution

An essential second action is the assessment of the redox behaviour of cocaine and the cutting agents in mixtures to elucidate potential masking (overlapping peak) sensations.

Square-wave voltammograms of 1 mM cocaine and also 0.5 mM cutting agent in a 0.1 M KCl + 0.020 M KH2PO4 solution (pH 7) at a bare carbon screen-printed electrode are presented in Fig. 3.


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Fig. 3 Baseline-corrected square-wave voltammetric responses (vs. Ag/AgCl) of 1 mM options of cocaine with 0.5 mM cutting agent at bare carbon screen-published electrodes in 0.1 M KCl + 0.020 M KH2PO4 buffer (pH 7). The dotted line represents the characteristic oxidation potential of cocaine at 1.04 V. The y-axis has the very same scale in eexceptionally voltammogram.

Tright here are no substantial top shifts visible for the cocaine signal and also the cutting agent peaks perform take place at their characteristic potential values. This peak potential of the cocaine oxidation process is just slightly shifted in the existence of benzocaine, levamisingle, caffeine, phenacetin, lidocaine, diltiazem and hydroxyzine with a worth of +18 mV, −12 mV, −32 mV, +13 mV, −17 mV, −7 mV and −32 mV respectively. As is clear from Fig. 2, the distinctions in between the oxidation potential of cocaine and also the redox potential of the cutting agents are much bigger compared to the small shifts observed when analysing mixtures. Benzocaine does, however, show more complex behaviour in mixture through cocaine. Although the mixture reflects both the characteristic peaks of cocaine and benzocaine, the voltammogram reveals an additional collection of peaks in the 0.25–0.45 V variety. These processes are not oboffered as soon as the individual components are measured independently and, therefore, it results from a certain interactivity in between cocaine and benzocaine, which is unknown at this allude. However, the set of peaks is well isolated from the cocaine signal itself and, therefore, gives additional information around the presence of cocaine and also benzocaine in a mixture. Minor second peaks are additionally visible at lower potentials for lidocaine, diltiazem and hydroxyzine, once in mixture with cocaine, and also are thus valuable for the assessment of the redox procedures in the mixtures.

Fig. 3 shows a partial overlap of the cocaine oxidation signal through the lidocaine oxidation signal. This slight overlap does not lead to a trouble for cocaine detection. The supplied concentration of lidocaine in this section, relative to cocaine, is a lot higher than the average content of lidocaine found in seized cocaine street samples. In 2014 and 2015, 13.5% of the seized cocaine samples, analysed by NICC making use of acattributed GC-MS and also GC-FID methods, had lidocaine with an average concentration of 5 wt%.

It is additionally clear from Fig. 3 that the signal intensity for cocaine drops significantly as soon as in mixture via levamisingle. This observation likewise uses to the levamisole signal. At this suggest, it is unclear what reasons this decrease in intensity for these components. Many importantly, cocaine might still be quickly detected. Due to the fact that we build a screening method without quantification, the reduced intensity develops no trouble for the application. The levamisingle content used in the experiments was once aacquire greater than the average content of levamisole in seized cocaine street samples. In 2014 and also 2015, 57% of the seized cocaine samples, analysed by NICC utilizing accredited GC-MS and GC-FID approaches, contained levamisole with an average concentration of 12 wt%.

Voltammetry of street samples in solution

In order to test the potential of the fingerprint method for the detection of cocaine in street samples, square-wave voltammograms of numerous seized street samples were videotaped in buffer solution (0.1 M KCl + 0.020 M KH2PO4 at pH 7) and also corrected for the background present as done before. All services were made such that they all had 1 mM of cocaine and the equivalent voltammograms are displayed in Fig. 4. Before the analysis, the qualitative and also quantitative complace of the street samples was determined at the National Institute of Criminalistics and also Criminology (NICC) using acattributed GC-MS and also GC-FID approaches.
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Fig. 4 Baseline-corrected square-wave voltammetric responses (vs. Ag/AgCl) of street samples at bare carbon screen-printed electrodes in 0.1 M KCl + 0.020 M KH2PO4 buffer solution (pH 7). The dotted line represents the characteristic oxidation potential of cocaine at 1.04 V. The y-axis has actually the exact same scale in eincredibly voltammogram. Each solution was ready in such means that it has 1 mM of cocaine.

Fig. 4 mirrors that the cocaine height is plainly detectable in street samples containing phenacetin, caffeine, levamisingle, boric acid, hydroxyzine and also lidocaine, also once the cocaine amount is only 30 wt% as is the situation in the third sample. Knowing that NICC in Belgium discovered that 95% of all seized cocaine samples in 2014 included more than 35 wt% of cocaine, this approach is incredibly promising to check out better for fast on-site screening. Even even more, based on the observations made in Fig. 2 and also 3, the existence of the cutting agents can be evidenced.

Two of the characteristic phenacetin peaks are clear and also sharp in the first sample at their characteristic potentials of 0.77 and also 0.35 V, whereas the signal for levamisole is likewise clearly visible at its characteristic potential of 1.27 V in the second sample. The second sample clearly mirrors the redox processes of cocaine and also levamisole at their fingerprint potentials, i.e. 1.04 and 1.27 V, respectively. The percentage cocaine is ca. 30% in the 3rd sample, however, clearly visible in the square-wave voltammogram. In the fourth sample we view a height at about 0.80 V, which is characteristic for hydroxyzine. Also a very little wave is visible at 0.10 V which is characteristic for either hydroxyzine or diltiazem. A small height is additionally visible at about 1.30 V, which shows the existence of caffeine. We were not able to detect the signal for lidocaine in this certain street sample, however this deserve to be defined by the extremely low concentration of the compound in the sample (Voltammeattempt of powder street samples For straightforward on-website usage, it is appropriate to detect the samples directly in its powder create, fairly than in solution, to protect against sample preparation. In order to execute so, we usage the on-site sweeping strategy, as is described in detail in the speculative area. A conducting gelatine hydrogel was offered as an electrolyte for the electrochemical measurements.

Square-wave voltammograms of numerous seized street samples were tape-recorded and also presented in Fig. 5, as well as a empty to retain the response of the gelatine gel itself and a sample only containing pure cocaine. A conditioning action was performed at 0 V rather than the previous preferred 1.5 V in solution, to optimize the sensitivity towards cocaine.


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Fig. 5 Baseline-corrected square-wave voltammetric responses (vs. Ag/AgCl) of powder street samples at bare carbon screen-printed electrodes making use of a gelatin gel B hydrogel containing 0.1 M KCl + 0.020 M KH2PO4 buffer solution (pH 7) as electrolyte. The dotted line represents the oxidation potential of cocaine at 0.97 V. The y-axis has the same range in eincredibly voltammogram.

Clearly on, the gel itself causes an intense electrochemical signal about 1.30 V and also a small optimal at 0.57 V. Any indevelopment at ca. 1.30 V will certainly, therefore, not be applicable to a certain cutting agent. However before, it is clear that this approach functions extremely well for rapid on-site cocaine detection. The cocaine optimal is very intense once pure cocaine is measured, as is clear from the second voltammogram in Fig. 5. The cocaine top is also well detectable for the street sample containing 23% levamisole and also for the sample via just 30% cocaine and an unrecognized amount of boric acid. A tiny transition in the height potentials is visible for powders entrapped in the gelatine matrix compared to the services. The cocaine height is now located at a potential of 0.97 V rather of 1.04 V.

This information-wealthy and single run square-wave voltammeattempt technique, utilizing electrodes immobilized on the fingertip of a glove, holds substantial promise for the rapid on-site screening of suspicious cargo and persons on the visibility of cocaine and also its cutting agents.

Reproducibility

The reproducibility of the derived signals in solution and also powders was stupassed away by perdeveloping 5 measurements, via each time a various SPE, on numerous of the earlier studied samples, all containing 1 mM cocaine. In solution we obtained results concerning the top potential of cocaine for pure cocaine and also three street samples. A pure cocaine solution showed the characteristic peak at 1.04 ± 0.02 V, while for a very first street sample containing 73% cocaine and 17% phenacetin, a 2nd street sample containing 76% cocaine, 3% caffeine, 10% hydroxyzine and much less than 1% lidocaine and also a third street sample containing 30% cocaine and also an unknown amount of boric acid confirmed optimal potentials of 1.06 ± 0.01 V, 1.03 ± 0.01 V and also 1.03 ± 0.01 V respectively.

The results display a maximal peak potential standard deviation of ±20 mV, likely led to by little distinctions in the internal referral electrodes in between various SPE"s. The potential shifts are, yet, tiny and also don"t cause any kind of troubles to recognize cocaine, likewise because these little shifts take place for the cutting agents too.

In powder create, the same experiment was percreated for pure cocaine, using the conductive gel as electrolyte and also the obtained top potential worth was 0.97 ± 0.01 V, showing the robustness and also reproducibility of the strategy. This stcapacity was assured by the rubber ring, immobilized on the thumb of the fingerguideline gadget as is explained in the speculative area. This ring keeps the conductive gel in location and it guarantees a constant applied pressure on the electrode surconfront while both fingers are joined in the time of the experiments, bring about secure baselines. The tiny potential variation additionally reflects that swiping the electrode surconfront over the powders does not cause a various electrochemical response caused by damaged electrode surencounters. This is one of the factors why graphite electrodes were used.

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Experimental

Reagents and also materials

The cocaine·HCl conventional was purchased from Lipomed (Arlesheim, Switzerland). Standards of phenacetin, diltiazem, lidocaine, procaine, hydroxyzine, benzocaine, paracetamol and myo-inositol were purchased from Sigma-Aldwealthy (Diegem, Belgium). Standards of benzoic acid and also levamisole were purchased from Acros Organics (Geel, Belgium). Standards of caffeine, boric acid, glucose, maltose and starch were purchased from VWR Chemicals (Leuven, Belgium) and a standard of D-sorbitol was purchased from Merck Chemicals KGaA (Overijse, Belgium). Authentic cocaine street samples were obtained from the National Institute for Criminalistics and Criminology (Brussels, Belgium). Gelatine gel B was offered by PB gelatins (United Kingdom). Carbon ItalSens IS-C Screen Printed Electrodes (SPE) were purchased from PalmSens (Utrecht, The Netherlands) and also were used throughout all electrochemical dimensions. The electrode surface area is 7.07 mm2. All laboratorium-based electrochemical dimensions were percreated making use of a Metrohm μAutolab III Potentiostat and also NOVA 1.11 software.

Synthesis of the conducting gel

A mixture of 2.5 wt% gelatine gel B in a 100 mM KCl and also 20 mM KH2PO4 buffer was heated in an Eppendorf tube to 50 °C for 15 minutes complied with by additional mixing till the solution ended up being homogeneous. The solution was then transferred to a syringe in which the hydrogel was created and also aged throughout the next 16 hours at room temperature before usage in the experiments.

Determicountry of samples in solution

The buffer solution used for the experiments was optimized to acquire maximal height separation and also sensitivity. The 0.1 M KCl + 0.020 M KH2PO4 buffer permitted the identification of cocaine without overlapping the signals of the cutting agents, as is displayed in Fig. 2. Solutions of 1 mM cocaine and all cutting agents were prepared in 0.1 M KCl + 0.02 M KH2PO4 buffer (pH 7) and stored at 4 °C before evaluation. Each solution was analyzed individually by placing a 50 μL drop on the SPE and square-wave voltammeattempt was perdeveloped in order to detect characteristic redox procedures for each substance. This was also performed for binary mixtures of cocaine and a cutting agent and also for authentic street samples.

On-website swiping method

Suspicious powders were analysed straight by utilizing a gelatine gel as a solid electrolyte. The SPE was put in a tiny SPE connector (with wires linked to the potentiostat) which was addressed on a steel ring. This ring is worn by the executer of the experiment on the index finger of a nitrile glove. The SPE was swiped gently over the suspicious powder in a manner which permitted some of the powder to be moved onto the working electrode of the SPE. 100 μL of the conductive gel was added with a syringe to a small reservoir on the thumb, surrounded by a tiny rubber ring. The rubber ring keeps the gel in area and also was immobilized on the thumb utilizing cyanoacrylate glue.

Both fingers were joined together, leading to completion of the electrochemical cell where upon the electrochemical measurement could be began. Each electrode and also percentage of gel was supplied just as soon as.

Square-wave voltammetry

Square-wave voltammeattempt (SWV) was perdeveloped to characterize the electrochemical fingerprint of cocaine (street) samples. A conditioning potential of 1.5 V was applied for 5 s, complied with by a 2nd conditioning action of 5 s at 0 V before a shave the right to from −0.1 V to a final potential of 1.5 V vs. Ag/AgCl was perdeveloped. All scans were percreated at a frequency of 10 Hz, via an amplitude of 25 mV, and also a action potential of 5 mV. For the experiments via powders rather of solution, a single conditioning step of 5 s at 0 V was perdeveloped.

Moving average iterative background correction

A baseline correction approach was constructed in the SWV procedure in the NOVA 1.11 software to automatically correct for the raising background present in order to make the voltammograms easier to interpret. In brief, the strategy compares the worth of a data allude Ai to the values of the previous and following data points Ai−1 and also Ai+1. If the worth of information suggest Ai is higher than the average of the values of points Ai−1 and also Ai+1 (as is the case for an oxidation peak), the average of the worths of Ai−1 and Ai+1 will rearea the value of Ai to construct the corrected baseline. In all various other instances once Ai is reduced or the exact same as the average of Ai−1 and Ai+1, Ai will be the value provided for the corrected baseline. This process was performed for each 2 data points in the voltammogram and also repetitive till the worth of Ai never before exceeds the average of the worths of Ai−1 and Ai+1 anyeven more, through a maximum of 1000 iterations. The corrected baseline is currently assembled and also the background present will certainly be zero. Confident currental fees are just visible at peaks of oxidation processes.

Conclusions

We have actually demonstrated the appliccapability of an electrochemical fingerprint approach to identify cocaine and also its cutting agents in street samples. Direct analysis with minimal sample preparation is possible many thanks to the integration of the screen-published electrode in a glove and also by using a conductive, flexible gelatine hydrogel as electrolyte. This new technique enables rapid on-website detection at points of interemainder such as airports and also harbours, by sindicate swiping the electrode device over the suspicious powder, join both fingers and also start the square-wave voltammetric measurement. Determination of the unique electrochemical fingerprint of both cocaine and the cutting agents in solution provides all vital analytical indevelopment to detect cocaine and also the cutting agents on-site in unwell-known suspicious samples. The brand-new concept thus holds significant promise as a portable, on-site applicable screening approach aimed at the rapid identification of cocaine/drugs (street) samples. To accomplish this, a switch from a huge potentiostat (favor μAutolab III) to a miniaturized, portable potentiostat device must be made. With the quick advancement of wiremuch less devices and communication, the transmission of results to a smartphone or a tablet would be of extensive energy to on-site drug investigations.

Acknowledgements

The authors acexpertise BELSPO for funding the APTADRU project (BR/314/PI/APTADRU).

Notes and also references