Use of an electrochemical room temperature ionic liquid-based microprobe for measurements in gaseous atmospheres

Highlights

• An electrochemical room temperature ionic liquid-based microprobe (RTIL-EMP) was prepared for gas analysis.

• The RTIL-EMP consisted of an integrated two electrode microcell.

• Two platinum microdiscs over micrometer dimensions were employed.

• A thin film of a room temperature ionic liquid acted as both solvent and supporting electrolyte.

• The performance of the RTIL-EMP, towards the detection of oxygen in air atmosphere, was satisfactory.

Abstract

A simple electrochemical microprobe (EMP) is proposed for the detection of analytes in gaseous atmospheres. The EMP consists of two platinum fibres of 25 and 300 μm in diameter encased into a theta glass pipette to form an electrochemical cell in a two-electrode configuration. Ion conductivity between the two electrodes is ensured by a thin film of the room temperature ionic liquid (RTIL), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, which is applied onto the EMP tip surface by a simple dip-coating procedure. The ionic liquid-coated microprobe (RTIL-EMP) was preliminarily investigated by using ferrocene as an electroactive species to ascertain the mass transport properties of the analytes that influence the voltammetric responses as well as the stability and reproducibility of the RTIL-EMP in the gas phase. The performance of the RTIL-EMP to gas analysis was afterward evaluated by using oxygen as electroactive species. The RTIL-EMP was exposed to different synthetic O₂/N₂ (v/v) mixtures and current responses were recorded as a function of O₂ concentration, using either cyclic voltammetry (CV) or chronoamperometry (CA). Regression analysis of the experimental current against % O₂ was linear over the range 0–100% with correlation coefficient and sensitivity of, respectively, 0.996 and 0.29 nA/(v/v) % O₂ in CV and 0.998 and 0.27 nA/(v/v) % O₂ in CA measurements. Long term stability, reproducibility of the RTIL-EMP recovery of the RTIL film layers to the initial conditions and effects of humidity on the current responses were investigated in detail.

Introduction

Electrochemical techniques and sensors for the detection of a great variety of substances have been extensively employed due to their high sensitivity, low cost, ease operation and good portability [1]. However, conventional electroanalytical devices, based on amperometric responses, cannot be directly applied to gaseous samples. To overcome this drawback, gas-permeable membrane electrodes have been developed, in which gaseous analytes can permeate the membrane and, after dissolution in an internal electrolyte, can diffuse to the working electrode surface [2], [3], [4]. Their performance is however conditioned by these slow steps because they cause lowering of sensitivity and lengthening of response time.

Remarkable benefits have been introduced by the use of gas sensors based on moist ion-exchange membranes, as solid polymer electrolytes (SPEs), in which the steps due to permeation and diffusion in solution are avoided [5], [6], [7], [8], [9], [10], [11], [12], [13]. Unfortunately, SPEs require the presence of an internal electrolyte, whose solvent can evaporate and cannot survive drastic temperature changes.

The past few years have seen the proposal of several gas electrochemical devices based on room temperature ionic liquids (RTILs) which act simultaneously as electrolytes and solvents [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. RTILs offer quite attractive physical properties since they are characterized by negligible vapour pressure, wide electrochemical windows, good thermal stability, inherent electrical conductivity, and tunability [27], [28], [29]. Due to the relatively high viscosity of RTILs, mass transport of redox species is usually much slower than in traditional aqueous or non aqueous electrolytes. Diffusion coefficients in RTILs are typically two or three orders of magnitude lower than those in conventional electrolytes [29]. This circumstance is unprofitable in electroanalytical measurements because slow diffusion coefficients lead to small current signals. However, the use of RTILs can still provide prominent advantages if they are placed as thin films directly onto an electrode surface. Various electrode surfaces have been suggested for RTILs membrane-free gas sensors, most of them are based on macrodisc electrodes [14], [15], [16], [17], [24], [25], [26], [30], [31] assembled in either conventional three-electrode electrochemical cells [14], [15], [16], [17], [24], [25], [26] or screen printed [30], [31]. A few examples of RTIL-gas sensors based on microelectrodes have also been reported in the literature [21], [22], [23]. In particular, platinum microdiscs [23], disposable microband electrodes [21], and arrays of recessed gold microdisc electrodes fabricated on a silicon chip by a standard photolithographic procedure [22] have been proposed. The use of microelectrodes coupled with RTILs provides the advantages of enhanced current densities, high faradic to capacitive current ratio and low ohmic drop [32], [33].

Here, we propose an alternative membrane-free microsensor for gas analysis based on a RTIL-coated microelectrode integrated with a pseudo reference electrode. The microcell is fabricated by using two Pt fibres of 25 μm and 300 μm in diameter, which are encased into a theta glass pipette to provide a two-disc electrodes tip. The tip end is coated by a thin film of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][NTF₂], which ensures ionic conductivity between the two electrodes. [BMIM][NTF₂] has been chosen as, at 20 °C, it displays a viscosity value equal to 50 mPa s [29], which allows keeping diffusivity of electroactive specie to acceptable low values, while allowing a good adhesion and stability of the film onto the tip surface. The performance of the microcell is firstly investigated in bulk RTIL containing ferrocene as model electrochemical probe for gaining general information on mass transport characteristics of the electroactive species. Secondly, it is applied to the detection of oxygen in gas phases. Overall, in this work we show that the proposed microcell can easily be assembled, the RTIL film can be quickly restored to its initial conditions for multiple measurements, and it can be used for continuous measurements even for several days in real world conditions.