Sponge-based microfluidic sampling for potentiometric ion sensing
Graphical abstract
Introduction
The analysis of ion concentrations in environmental and clinical samples are commonly conducted, however, on-site analysis still faces many challenges, such as need of sample pre-manipulations and application of a non-portable analytical instrumentation [[1], [2], [3]]. By applying on-site sensing the reduction in the time delay between sampling and measurements can be realized through more accurate sampling that is integrated with the on-site detection of the analyte [4]. For example, inductively coupled plasma mass spectrometry (ICP-MS), requires laborious sample pre-treatment e.g. filtration, dilution or pre-concentration of the analyte [[5], [6], [7], [8]]. Moreover, samples that contain volumes lesser than milliliter usually cannot be analyzed without advanced sampling and sample handling devices [9,10].
The ion-selective electrodes (ISEs) have been extensively studied in context of on-site analysis of ions [[11], [12], [13], [14], [15], [16]]. ISEs are portable and relatively low-cost ion sensors, which makes them adaptable for on-site sensing. Their potentiometric response majorly depends on the physico-chemical state of an ion-selective membrane (ISM) and the protocol of the analysis [17,18]. In order to measure analytes in sub milliliter sample volumes, furthermore to avoid processing of sample through application of filtration and to maintain the mechanical integrity of the ISM, microfluidic analyte sampling was developed and integrated with the ISEs. The microfluidic analyte sampling was realized through application of paper and textile as sampling and sample handling substrates [[19], [20], [21], [22]]. The measurements of ions applying microfluidic paper- and textile-based sampling, such as sodium, potassium and chloride were found comparable to the ones done in conventional, beaker-based measurements. On the other hand, measurement of pH and heavy metals were found altered by the presence of paper and textile in direct contact with the standard and sample solutions. The pH of the sampling substrate, resulting from the chemical nature or manufacturing process of the substrate, influenced the pH when in contact with the sample solution [23]. In heavy metal analysis, the super Nernstian response of ISEs was observed when applying both types of microfluidic sampling. This effect can be explained by the physical/chemical adsorption of heavy metal ions onto substrates, causing depletion of ion concentration at the ISM surface [19,20]. To address this unfavourable effect, we have developed recently a method to diminish the super-Nernstian response by pre-treatment of paper substrates in various concentrated solutions of primary ion, prior to the actual potentiometric measurements [4]. Nonetheless, a further search for a more suitable substrates that could be applied directly for the microfluidic analyte sampling and sample handling is necessary to assure reliable determination of ions in environmental and clinical samples.
A sponge is a material that can be used for wicking, thus delivering the solution to the sensor surface for the detection of ions in small volume with high impurity content samples. There are various types of sponge materials, e.g. natural (sea sponges) and synthetic (polyurethane (PU), polyester and cellulose). Sponges are low cost materials that have excellent liquid wicking ability [[24], [25], [26]]. For example, they can be used for selective removal of oil and toxic contaminants in oil spills and industrial discharge. Also, they can be reused after the wicked substance is pressed out under the mechanical stress [[27], [28], [29], [30], [31]].
In this work, microfluidic sponge-based sampling was studied as novel sampling and sample handling method to serve as alternative for microfluidic paper- and textile-based samplings for application in ion sensing. For that reason, the microfluidic sponge-based sampling was integrated with ISEs and its performance was evaluated in the context of possible measurements in a clinical and an environmental samples.
Section snippets
Chemicals, materials and electrodes
Potassium chloride (KCl) (purity ≥ 99%) was obtained from Merck KGaA (Germany), sodium chloride (NaCl) (purity ≥ 99%), potassium hexacyanoferrate (III) (purity ≥ 99%) were purchased from VWR International (USA). 70% HNO3 (purity ≥ 99%, trace metal grade), cadmium nitrate (Cd(NO3)2) (purity ≥ 98%), lead nitrate (Pb(NO3)2) (purity ≥ 99%), sodium polystyrene sulfonate (NaPSS) (purity ≥ 99%), 3,4-ethylenedioxythiophene (EDOT) (purity ≥ 99%), potassium ionophore I (purity ≥ 99%), lead ionophore IV
Characterization of sponges and sponge-based microfluidic sampling
The physico-chemical characterization of each type of sponges is shown in Table 1. Basic parameters of each sponge, such as the liquid absorption capacity, the evaporation rate and pH were investigated. The PU sponge had the highest liquid absorption capacity, while the cellulose and the natural sponges had similar and at least more than 3 times lower liquid absorption capacity than PU sponge. The PU sponge substrates had more porous structure and lower density (11 kg m−3) than other cellulose
Conclusion
This work opens new area of material use for microfluidic solution sampling, e.g. application of polyurethane sponge substrates. The microfluidic sponge-based sampling was developed and studied as novel sampling and sample handling substrate to serve as alternative for microfluidic paper- and textile-based solution sampling for ion analysis in various clinical and environmental samples. The microfluidic sampling allows measurements of ions in samples with low volume of liquids and high content
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.
Acknowledgements
The authors would like to acknowledge the Economic Development Board (Singapore) for financial support of this research.
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