Elsevier

Talanta

Volume 224, 1 March 2021, 121732
Talanta

Electrochemical micropipette-tip for low-cost environmental applications: Determination of anionic surfactants through their interaction with methylene blue

https://doi.org/10.1016/j.talanta.2020.121732Get rights and content

Highlights

  • Pin-based electrochemical cell in a multifunctional micropipette tip was developed.

  • Low-cost electroanalysis using stainless-steel pins and micropipette tips was achieved.

  • Fast determination of surfactants using a micropipette-tip-based electrochemical cell.

  • Electrochemical determination of SDS based on its interaction with methylene blue.

  • Water analysis was performed using a micropipette-tip based electrochemical cell.

Abstract

Miniaturization is one of the main requirements in the design of portable devices that allow in-field analysis. This is especially interesting in environmental monitoring, where the time of the sample-to-result process could be decreased considerably by approaching the analytical platforms to the sampling point. We employed traditional mass-produced and low-cost elements (micropipette tips and pins) in an out-of-box application to generate an innovative and cost-effective platform for analytical purposes. We have designed simple and easy-to-use electrochemical cells inside polypropylene micropipette tips with three stainless-steel pins acting as the working, reference and counter electrodes of a potentiostatic system. The pin acting as working electrode was previously coated with carbon ink, meanwhile the rest were used unmodified. In this way, electrochemical in-the-tip measurements were done directly using low volumes (μL) of sample. The devices showed good reproducibility, with a relative standard deviation of 7% (n = 5) for five different tip-based complete electrochemical cells. As a proof-of-concept, its utility has been probed by the determination of an anionic surfactant (sodium dodecyl sulphate, SDS) in water through its interaction with methylene blue (MB). Two different alternatives were presented based on the: 1) increase in the current intensity of the cathodic peak of MB due to the presence of SDS; 2) electrochemical determination of the MB remaining in the aqueous phase after extraction of the pair SDS-MB to an organic medium.

Introduction

The design of miniaturized and low-cost sensing devices is an area of enormous interest in the field of Analytical Chemistry. They are definitely required to move (bio)chemical analysis from the bench to the field [1]. In fact, the combination of wireless sensors with smartphones is producing platforms well suited to perform decentralized analysis [2,3]. In this context, the electrochemical detection must be pointed out since it is low cost and easily miniaturized, and provide highly selective and sensitive methodologies [4].

In terms of cost-effectiveness, the use of common elements for out-of-box applications is clearly increasing. They are already mass-produced items that can be utilized for novel applications. In 2016, Whiteside's group introduced the use of stainless-steel pins as electrodes in combination with thread for microfluidic determination of lactate [5]. The modification of these metallic elements with carbon ink allowed their use to determine glucose enzymatically, in static [6] and flow-based systems [7]. Batch injection analysis has also been demonstrated for epinephrine determination using pin-based cells [8]. Their versatility and low size enabled the design of arrays for performing simultaneous measurements using a multichannel potentiostat [6]. Similarly, other mass-produced elements of common use such as staples have been modified for utilising them as electrodes in new electrochemical low-cost platforms [9].

Among the items that are commonly present in most (bio)chemical laboratories and can be employed in out-of-box applications are micropipette tips. They can be used for different purposes apart from dispensing liquids, their main function. These novel applications include sample purification, as in the case of tips containing a bed of chromatography media for the detection of human chorionic gonadotropin (hCG) and prostate-specific antigen (PSA) in urine samples [10], or selective extraction, with integrated molecularly-imprinted monoliths, of the toxin berberine [11] or the pesticide methomyl [12]. More recently, our research group designed an enzymatic immunoassay in which the inner surface of a micropipette tip was turned into a capture zone for anti-tissue transglutaminase (IgA) [13], antigen of interest for the diagnosis of the celiac disease.

Moving further in this direction, in this work we have added a new functionality to this common and affordable consumable material, easily found in laboratories. With this aim, we have integrated a very simple pin-based electrochemical cell inside a micropipette tip. Already in 1991, a micropipette tip was used as a platform for the fabrication of carbon fiber ultramicroelectrodes [14]. Following this idea of using the tip as container, miniaturized electrochemical devices were designed using micropipette tips combined with e.g., graphite composite [15] or epoxy resin [16]. In these examples, the micropipette tip acted as the body of the working electrode, that had to be immersed in the sample solution for performing measurements, as happens with e.g., a polymeric cylinder that encloses carbon paste or a pencil lead. The other two electrodes (reference and counter electrodes) had to be introduced in the solution as well, which could be done: i) independently, as different elements, or ii) integrated in the outer part or the tip. Apart from these approaches, the tips were used, after sealing the bottom, as low-cost and low-volume electrochemical cells. Following this, three small-size electrodes with a pencil graphite working electrode were introduced in a tip-based cell [17]. However, in all these cases the tip was modified and could not be used in combination with a micropipette for performing its original function: aspirating and dispensing solutions. There is a unique report on the use of a micropipette-tip as container of a complete electrochemical cell retaining its function [18]. In that case, the sample was aspirated and analyzed thanks to the introduction of three wire electrodes (with gold-based working and reference electrodes) and a cotton wool filter (for medium storage and sample purification) inside the tip, in a new lab-on-a-tip approach for decentralized electroanalysis of Cu2+ in water.

In this work, we propose the use of a simpler and cheaper device that employs stainless-steel pins as electrodes and integrates them in the micropipette tip, which becomes now multifunctional. A pin modified with carbon ink is used as working electrode, and two unmodified pins are used as reference and counter electrodes. One important advantage is that the modification of pins with carbon ink allows their use in many applications. Carbon materials, such as carbon paste/ink and graphite, are widely used in electrochemistry because of their advantageous properties: carbon is cheap and chemically inert, and shows high surface area and electrical conductivity, low background currents and a wide potential window [19,20]. The bottom of the tip has a diameter that allows the introduction of the pin covered with a thick film of carbon, with the head of the pin acting as stopper. Following this new design, the electrochemical cell can be fabricated in an extremely fast and reproducible way employing inexpensive materials.

Thus, we present here a modified micropipette tip that acts as electrochemical cell, very useful for in-field analysis. The combination of common (stainless-steel pins) and lab (micropipette tips) materials described here draw us to the idea of a simple “everywhere” electroanalysis that could be performed by non-qualified personal. The portability of the system makes it useful for environmental analysis, e.g. on-site analysis of water and wastewater. Then, we decided to evaluate this new electrochemical lab-on-a-tip device for assessing the quality of water in terms of the presence of anionic surfactants. They mainly come from household aqueous discharge, industrial laundering and other cleansing operations. Surfactants are considered emerging contaminants. They have negative effects on water quality since they tend to congregate providing properties such as foaming, emulsification and particle suspension. Moreover, surfactants can pass through wastewater treatment plants and reach environmental and even drinking water. These compounds produce harmful effects in aquatic flora and fauna, and also in human health [21]. Therefore, the concentration of surfactant in water is an important parameter for assessing water quality. In the United States, ionic surfactants (mostly anionic) are about two thirds of the total surfactants used [22]. The standard analytical method highlights anionic surfactants, such as sodium dodecyl sulphate (SDS), as the most common substances present in water and it is based in their methylene blue (MB) activity. Anionic surfactants interact with MB, a cationic dye, forming an ionic pair. This complex is transferred from the aqueous solution into an immiscible organic solvent upon equilibration. The blue color intensity obtained in the organic phase is measured spectrophotometrically and is proportional to the concentration of anionic surfactants. This interaction has also been applied inversely to adsorb MB (dye discharged to the environment from textiles, dyeing and printing industries) on SDS-modified zeolites [23]. We have applied a modification of this standard analytical method in which a reduction in the sample volume and, henceforth, in the use of reagents, is achieved [24].

Moreover, instead of using a colorimetric assay, we have taken advantage of the electrochemical activity of MB and its growing interest in biosensing (e.g. as indicator of DNA hybridization processes through its interaction with the double strand [25]). Then, we have adapted the modified standard method to detect electrochemically MB, directly in the micropipette tip, as a way of determining SDS, a MBAS (methylene blue active substance) frequently found in wastewaters. Then, when a constant concentration of MB is added to the sample, its residual concentration in the aqueous phase, after interaction with MBAS and further extraction to an organic media, is indicative of the initial MBAS concentration in the sample.

Additionally, and considering the influence of surfactants on carbon electrodes, which can improve electrochemical processes [[26], [27], [28]], a simplified electrochemical method for the determination of SDS without following the extraction protocol previously indicated, was also developed. In this way, an increase in the electrochemical signal of MB (fixed concentration), seen for increasing SDS concentrations, is the basis for its determination. Then, in this work, we report two simple alternatives for SDS determination in water samples using an innovative and low-cost multifunctional electrochemical platform.

Section snippets

Chemicals and materials

Through all this work, 100–1000 μL polypropylene micropipette tips were used (Labbox Labware). For the assembly of the system and to facilitate micropipette handling, a foot stand with a three-point gripper fastening tong was employed. As electrodes, 26 × 0.59 mm stainless-steel pins (Metalurgica Folch) were selected. Carbon paste (Gwent Group Ref. C10903P14) and N,N-dimethylformamide (DMF, Merck) were employed for the preparation of the carbon ink, used in the working electrode. Acidic

Electrochemical design and characterization of the platform

The design of the electrochemical cell is based on the use of very cheap and available elements: pins and tips. Micropipette tips are very versatile in regard to the size (volume) and materials. In this case we chose commercial polypropylene tips of 100–1000 μL volume, but a different polymer could be employed (even modified) and pins could be inserted in tips of lower/higher volume to adjust the volume of the electrochemical cell according to the requirements of the application. Pins are

Conclusions

In-field analysis needs portable and simple devices and electroanalytical platforms have demonstrated to be very appropriate. In this context, a new electrochemical cell was fabricated in a very simple way using a micropipette tip and stainless-steel pins. Its good electroanalytical performance was demonstrated using methylene blue as known redox probe. Moreover, this novel tip-based cell was applied to real environmental analysis. It was applied to anionic surfactant determination, very

Credit author statement

A.González-López: methodology, investigation, visualization, writing original draft; E. Costa-Rama: methodology, supervision, writing-review and editing; M.T. Fernández-Abedul: conceptualization, methodology, supervision, writing-review and editing.

Author contributions

The manuscript was written through contributions of all authors.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

Andrea González-López thanks the Consejería de Educación y Cultura del Gobierno del Principado de Asturias for the award of her “Severo Ochoa” grant (BP17-36).

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