Novel hydrazinium polyacrylate-based electrochemical gas sensor for formaldehyde

Abstract

A disposable electrochemical sensor was developed for convenient detection of gaseous formaldehyde. The sensor is composed of a three-electrode screen printed system modified with hydrazinium polyacrylate which was synthesised to serve a dual purpose, i.e. as formaldehyde accumulation/derivatisation medium, and as polyelectrolyte enabling voltammetric measurements. It exhibited a linear response in the examined concentration range of 4–16 ppm gaseous formaldehyde in combination with 120 min accumulation; also sub-ppm concentration levels could be detected when the exposure time was extended.

Introduction

Formaldehyde is considered as one of the most important (indoor) pollutants due to its high toxicity and potential carcinogenicity. Gaseous formaldehyde originates from a variety of different sources, such as industry (e.g. resin production), traffic, forest fires, and tobacco smoke. It can also be emitted from different objects, e.g. new furniture and polymeric materials undergoing the degradation process [1], [2], [3], thus it can be used as a marker molecule for material degradation.

The most common analytical method used for formaldehyde detection is its derivatisation with 2,4-dinitrophenylhydrazine combined with high-performance liquid chromatography and UV detection [4]. For gas-phase analysis formaldehyde is first collected at different sorptive media (e.g. in impingers, denuders or on solid sorbents), which contain or are impregnated with the derivatising agent [5]. Although this method is reliable and precise, it requires relatively expensive instrumentation and can only be performed in a laboratory environment. The development of novel formaldehyde detection approaches and sensors thus presents an attractive topic. The already existing sensors vary in the measurement principle ranging from spectrometric, piezoresistive and colorimetric to amperometric and conductometric [6], the latter being the most widely investigated; metal oxide films/nanostructured metal oxides are commonly used in this case as sensing layers. Although relatively high sensitivity can be achieved using this methodology, its inherent drawbacks are the need for elevated temperatures and difficulties associated with measurements in humid environments [7], [8]. To overcome the operation at high temperatures, conductive polymer-based sensing layers were introduced for detecting volatile organic compounds (VOCs), e.g. formaldehyde. Upon interaction with the polymer, the analyte alters the polymer's physical properties, manifesting in the change of conductivity or in the shift of transmission spectrum [8].

The formaldehyde gas sensors of different types, e.g. amperometric, conductometric and colorimetric, often involve the enzyme formaldehyde dehydrogenase [9], [10], [11], [12], [13]; however, due to potential issues with the stability of enzyme-based sensors, their use is not always appropriate for all environments/applications. The enzyme-free electrochemical sensors with improved performance are therefore emerging [14], [15], although voltammetric formaldehyde gas sensors are still very rare. An interesting example was introduced by Gębicki [14], using a platinum working electrode, ionic liquid as the electrolyte, and a polydimethylsiloxane membrane to constitute a prototype gas sensor for formaldehyde.

In this work a new concept of a multipurpose polymeric sensing layer for room-temperature voltammetric detection of gaseous formaldehyde is introduced. A synthesised polymeric modification/sensing coating, which combines analyte accumulation capacity and its derivatisation via a well-known hydrazine-based reaction, and at the same time acts as the polymeric electrolyte enabling voltammetric measurements, was developed and studied to prepare a simple and inexpensive (disposable) sensor for gaseous formaldehyde detection.