Carbon Nano-Onion Modified Electrodes for Voltammetric Detection of Propellent Stabiliser 1,3-Diethyl-1,3-Diphenly Urea (Centralite)
Authors: McKeever Colm , Murphy Eoghian , Camisasca Adalberto , Giordani Siliva , Dempsey Eithne 
Published: July 2021

The testing of gunshot residue (GSR) has been a fundamental pillar of forensic analysis in the evaluation of evidence and its subsequent use in criminal trials involving firearms. While confirmatory testing of the inorganic fraction of gunshot residue is largely laboratory confined and reliant on scanning electron microscopy for particle analysis and inductively coupled plasma – MS for bulk metal determination, the move to onsite testing of this fraction has been facilitated using electroanalysis. Complementary recovery and analysis of the organic fraction of GSR remains an analytical challenge at the point of use. Such a device would provide more robust dual information due to the low environmental background levels of propellant stabilisers such as diphenylamine (DPA), and urea derivative 1,3-diethyl-1, 3-diphenylurea (Centralite). Electroanalysis of the latter is the key focus of this work, ultimately utilising screen printed electodes functionalised with electrocatalytic materials which serve to enhance the analytical signal.

An initial electrochemical examination of Centralite involved voltammetric studies in both aqeuous and non-aqueous environments, with the view to establish an understanding of its redox solution behaviour, pH effects, chemical stability and film formation upon potential sweeping. This allowed both mechanistic and analytical evaluation with the aim to achieve the required selectivity and sensitivity for reliable detection. Centralite electrochemistry was examined at glassy carbon electrodes in both aqueous (3:7 methanol:sodium acetate pH 4.3) and non–aqueous (0.1 M LiClO4 in methanol) electrolytes via potential sweeping. In the case of the aqueous system an irreversible wave at Ep = 1.2 V associated with a weak cathodic signal at 1.0 V vs. Ag/AgCl was observed. This diffusion controlled process resulted in diffusion co-efficient (D)=7.4x10-5 cm2s-1 which, for a one electron process results in transfer coefficient (a)=0.724. Upon cycling, a new anodic signal appeared at 1.0 V with a corresponding decrease in the process at 1.2 V vs. Ag/AgCl. Scan rate studies indicated a mixed diffusion/adsorption process with peak current ratios (anodic/cathodic) decreasing with respect to scan rate. In the acidic environment acid hydrolysis of the 1,3-diethyl-1, 3-diphenylurea can result in N-ethylaniline and N-ethyl-N-phenylformamide (a type of carbamic acid which can revert back to the parent amine with release of CO2). Deposition of poly(N-ethylaniline) could explain the presence of a clearly visible yellow film on the electrode surface (Fig. 1). Upon examination of Centralite in 0.1 M LiClO4 in acetonitrile an irreversible process at 1 V was evident with a cathodic wave at 0.71 V and the appearance of a new anodic peak at 0.75 V vs. Ag/Ag+. Scan rate studies revealed that these processes were diffusion controlled.

The electrochemical behaviour of carbon nanomaterials including their capacitance can depend on counter ion film penetration, the double layer structure and electrolyte. Recently, carbon nano-onions (CNOs), an emerging member of the carbon family, have shown promising electrochemical performance towards the oxygen reduction reaction as a consequence of a co-doping process.

Herein, both pristine (p-) and boron nitrogen co-doped (BN-) CNOs were examined as solid films which were dropcast onto glassy carbon electrodes from a dispersion of 1 mg/mL in ethanol. There was a capacitive and faradaic component to the current and good mechanical stability was evident upon potential cycling. In the case of BN-CNOs, a quasi-reversible surface confined process was evident at Ep(a) = 0.27 V and Ep(c) = 0.057 vs. Ag/AgCl with DEp = 0.213 V. When examined in an anionic redox probe [Fe(CN)6]3-/4- a two fold decrease in DEp was evident (indicating faster charge transfer at the CNO modified surface). Surface coverage corresponded to 6.73 - 5.44 x 10-10 mol cm-2 using current vs scan rate plots. Specific capacitance was 16.96 F/g and 3.62 F/g for pristine and BN doped CNO material respectively being scan rate dependent. Centralite calibration studies, in the aqueous environment using linear sweep voltammetry resulted in sensitivity values of 0.585 A.cm-1.M-1 for the bare glassy carbon electrode and 0.710 A.cm-1.M-1 in the case of the a CNO modified electrode with further work ongoing.

Figure 1