Sponge-based microfluidic sampling for potentiometric ion sensing

Analytica Chimica Acta 1091 (2019) 103e111

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Sponge-based microfluidic sampling for potentiometric ion sensing Ruiyu Ding a, Grzegorz Lisak a, b, * a

College of Engineering, School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798, Singapore b Nanyang Environment and Water Research Institute, Residues and Resource Reclamation Center, 1 Cleantech Loop, Cleantech, Singapore, 637141, Singapore

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Sponge-based microfluidic sampling coupled with potentiometric sensors.
Direct application of sponge-based microfluidic sampling in heavy metal analysis.
Determination of ions in clinical and environmental samples.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 July 2019 Received in revised form 19 August 2019 Accepted 9 September 2019 Available online 11 September 2019

This work demonstrates an application of the new materials, sponges, for the use in microfluidic solution sampling integrated with ion-selective electrodes. The microfluidic sponge-based sampling was developed and studied as novel sampling and sample handling method to serve as alternative for microfluidic paper- and textile-based sampling for ion analysis in various environmental (Cd2þ, Pb2þ and pH) and clinically (Kþ, Naþ, Cl) relevant samples. Three types of polyurethane, cellulose and natural sponges were used as substrates for microfluidic solution sampling. Polyurethane based sponge was found to have low heavy metal sorption capacity thus it was recognized as suitable for microfluidic sampling coupled with solid-state as well as solid-contact ion-selective electrodes. The application of spongebased microfluidic sampling, contrary to previous findings of paper- and textile-based microfluidic sampling, allowed measurements of heavy metals without prior modification of the sampling substrate. Finally, the determination of potassium, sodium and chloride in wastewater sludge and sweat samples done with sponge-based microfluidic sampling integrated with ISEs was found similar to analysis done by ICP-OES and IC. © 2019 Published by Elsevier B.V.

Keywords: Microfluidic sponge-based sampling Potentiometric ion sensing Heavy metal analysis Clinical and environmental samples

1. Introduction

* Corresponding author. College of Engineering, School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798, Singapore. E-mail address: [email protected] (G. Lisak). https://doi.org/10.1016/j.aca.2019.09.024 0003-2670/© 2019 Published by Elsevier B.V.

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 [1e3]. By applying on-site sensing the reduction in the time delay

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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 pretreatment e.g. filtration, dilution or pre-concentration of the analyte [5e8]. 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 [11e16]. 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 [19e22]. 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 pretreatment 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 [24e26]. 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 [27e31]. 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 spongebased sampling was integrated with ISEs and its performance was evaluated in the context of possible measurements in a clinical and an environmental samples. 2. Experimental 2.1. 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 (purity  99%), sodium ionophore X (purity  99%), bis(2ethylhexyl) sebacate (DOS) (purity  99%), tridodecylmethylammonium chloride (TDMACl) (purity  99%), potassium tetrakis(4echlorophenyl) borate (KTpClPB) (purity  99%), 2Nitrophenyl octylether (o-NPOE) (purity  99%), poly(vinyl chloride), (PVC) (purity  99%) and tetrahydrofuran (THF) (purity  97%) were obtained from Sigma-Aldrich (Germany). The instrument calibration standard 2 used for measurements done with inductively coupled plasma-optical emission spectrometry (ICP-OES) was obtained from PerkinElmer, Inc. (USA). An IV-STOCK59 for ion chromatography (IC) standards was purchased from Inorganic Ventures (US). The polyurethane (PU) sponges were purchased from YongSheng sponge plant (China). The natural sea sponges were obtained as commercial product from Netease Yanxuan (China). The cellulose sponges were purchased from 3 M (USA). The technical pH buffer solutions, the InLab Surface Pro-ISM pH electrode (flat surface) and SG23 pH meter were obtained from Mettler-Toledo (Switzerland). The crystalline solid-state Cd2þ-ISE and the single-junction Ag/AgCl (3 M KCl) reference electrode were purchased from Thermo Fisher (USA). Glassy carbon (GC) disk electrodes were obtained from Bioanalytical Systems (USA). The ultra-pure water of 18 MU cm was used to prepare all solutions. The ultra-pure water was obtained with the Milli-Q Integral Water Purification System (USA). 2.2. Preparation of sponge substrates and electrodes The sponge substrates were cut from original material into pieces of 3  2  0.5 cm3 and soaked in ultra-pure water for 30 min while stirring to remove any impurities that were water-soluble. Subsequently, sponge substrates subjected to drying in oven at 70  C for 30 min and were deemed to be ready for microfluidic sampling of analytes. Before each series of potentiometric measurements, the surface of the commercial solid-state crystalline Cd2þ- ISE was renewed by polishing the electrode on a soft pad using 0.05 mm alumina slurry (Al2O3) and subsequently rinsing it with ultra-pure water. The cadmium-selective electrode, between different series of potentiometric measurements, was stored in the 103 M Cd(NO3)2. Then the solid-contact Pb2þ-, Kþ-, Naþ-, Cl-ISEs were prepared by firstly polishing GC electrodes on a soft pad using 0.05 mm alumina slurry and subsequently rinsing the electrodes with ultra-pure water. Then, the electropolymerization of PEDOT doped either by PSS for Pb2þ-, Kþ-, Naþ-ISEs or Cl for Cl-ISEs on previously polished GC electrodes was performed in an electrochemical cell consisting of three electrodes in the presence of 0.01 M EDOT and 0.1 M NaPSS or KCl (for cation and anion sensitive ISEs, respectively). In the electrochemical cell, the GC electrodes, a mash made of Pt and the Ag/ AgCl (3 M KCl) were used as a working, a counter and a reference electrodes, respectively. The electropolymerization was carried out by passing through the working and counter electrodes a constant current equivalent to the current density of 0.2 mA cm2 for 714s, which was performed by CHI760E electrochemical workstation, CH Instruments, Inc. (USA) [32,33]. After completion of electropolymerization, the electrodes again were rinsed with ultra-pure water and left to dry in open air at the room temperature (23e25  C) for 12 h. Then, the ISMs were drop-cast on PEDOT (PSS or Cl) films. The applied ion selective membrane cocktails were (w/w %): 1% lead ionophore IV, 0.5% KTpClPB, 65.2% o-NPOE and

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33.3% PVC for Pb2þ-ISEs, 1% potassium ionophore I, 0.3% KTpClPB, 65.2% DOS and 33% PVC for Kþ-ISEs, 1% sodium ionophore X, 0.3% KTpClPB, 66% o-NPOE and 32.7% PVC for Naþ-ISEs, and 15% TDMACl, 51% o-NPOE and 34% PVC for Cl-ISEs. The cocktail components were dissolved in THF (2 mL) for every ISM. The membrane cocktail was drop-cast onto the electrode in portions of 20 mL with a 1-h interval between each portion applied, to reach for every electrode a total of 60 mL ISM cocktail. The electrodes were at room temperature (23e25  C) in open air for 12 h before the use. The potentiometric measurements were performed on conditioned ISEs. The conditioning was done for 24 h in 103 M Pb(NO3)2 for Pb2þ-ISEs, 103 M KCl for Kþ-and Cl-ISEs and 103 M NaCl for NaþISEs. The ISEs, between measurements, were stored in their respective conditioning solutions. 2.3. Physico-chemical characterization of sponges and spongebased microfluidic sampling coupled with Cd2þ-, Pb2þ-, Kþ-, Naþ-, Cl-ISEs. A maximum liquid wicking capacity of each sponge was tested by the mass change before and after soaking of a 3  2  0.5 cm3 piece of sponge in ultra-pure water. An evaporation rate of liquid registered for each sponge was obtained by calculating the change of mass between the sponge reaching its maximum wicking capability and 60 s after being exposed to evaporation in the open air at room temperature (23e25  C). The pH of sponges was carried out by placing the flat surface pH electrode on the top of a sponge substrate after reaching its full wicking capacity with ultra-pure water. The density of the sponges was estimated by looking at mass of cut 1  1  1 cm3 cubes of dry sponges. The morphology of PU sponge before and after microfluidic sampling done on wastewater sludge taken from Water Reclamation Plant (UPWRP) Singapore was investigated by JSM-7200F field emission scanning electron microscopy (FESEM) (Japan). After PU sponge was applied for microfluidic solution sampling in wastewater sludge, the substrate was left to dry overnight in the open air at room temperature (23e25  C). Then, the sponge substrate was cut into half to expose the clear cross section and two positions on that cross sections were investigated by FESEM. The potentiometric response of ISEs was measured using an EMF16 interface potentiometer (Lawson Labs Inc., USA). The singlejunction, Ag/AgCl (3 M KCl), was applied as the reference electrode. The potential of the ISEs was always measured by increasing tenfold the concentration of the primary ion in the standard solutions. All the calibration curves were obtained from 107 to 101 M. For the calibration curves, the potential was taken after 60 s of the measurement in each solution. In beaker-based potentiometric measurements, the potentiometric cell was immersed in the standard/sample solutions. Schematic representation of potentiometric cell integrated with sponge-based microfluidic solution sampling is shown in Fig. 1. In the microfluidic, sponge-based potentiometric measurements the sponge substrate was first immersed into standard solutions or pressed directly against environmental/clinical samples. The actual solution sampling was done in approx. 5e10 s, where the standard or sample solutions were wicked into the matrix of the sponge substrate. Moreover, in our previous study, utilizing microfluidic paper-based sampling [4], it was identified that the potentiometric cell response is not influenced by force applied between electrodes and the sampling substrates. Thus no additional control of force between electrodes and the sampling substrates during potentiometric measurements was used and the potentiometric cell was simply placed onto the sponge substrates, pressing the sponge substrate with gravitational force. The potential in each standard solution was measured for 60 s. A new sponge substrate was used for each potential

Fig. 1. Schematic representation of potentiometric cell utilizing sponge-based microfluidic sampling.

measurement. The activity coefficients were estimated by DebyeHückel approximation. Three consecutive measurement for each sponge in each solution were performed (n ¼ 3). All experiments were done at room temperature (23e25  C). 2.4. Heavy metals sorption onto the sponge substrates The sponge substrates were soaked in 106, 105, 104 and 103 M solutions of Cd(NO3)2 or Pb(NO3)2 for 30 min. After soaking, sponge substrates were taken out and vigorously rinsed with ultrapure water and subjected to drying in the oven at 70  C for 30 min. Then, each sponge substrate was weighted and cut into small pieces and subsequently placed into a 100 mL Teflon vessel. The sample digestion, in presence of 70% HNO3 (trace metal grade), was done in Titan microwave sample preparation system (MPS) (USA). Then, the microwave digestion of sponges substrates was performed as follow: (i) 5 min at 120  C and 30 bar, (ii) the temperature was raised to 170  C while keeping pressure constant and kept at this temperature for another 5 min, (iii) the temperature was decreased to 50  C and kept for 10 min and (iv) the samples were left to slowly cool down [4]. Subsequently, each sample after digestion was diluted and filtered by Acrodisc 25 mm syringe filter with 0.45 mm supor membrane (Pall, USA). The concentrations of lead and cadmium in the obtained samples were determined by PerkinElmer Optima 8300 inductively coupled plasma optical emission spectrometry (ICP-OES) (USA). 2.5. Analytical determination of pH, Kþ, Naþ, Cl in environmental and clinical samples The environmental samples were wastewater sludge taken from Water Reclamation Plant (UPWRP) Singapore, wet soil collected on NTU campus after the rainfall and mud from a water puddle of Sungei Buloh Wetland Reserve, Singapore. The clinical samples, namely sweat samples (further referred to as sweat sample A and B) were collected from a human subject exposed to physical stress (jogging). At least 3 mL of sweat samples were collected in the glass vial. The samples were freshly collected for the analysis. Prior to pH measurements, the calibration of the pH meter (SG23) was done using three buffer solutions. Beaker based pH measurements were carried out in liquid fraction of collected samples after filtration done by Acrodisc 25 mm Syringe Filter with 0.45 mm supor membrane (Pall, USA). In pH measurements, the sponge substrates were

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put onto the unprocessed sample to wick the liquid while leaving major solids behind. A pH electrode (flat surface) was placed onto the sponge containing wicked sample. The pH measurement was run until stable pH response was attained by the pH meter (approx. 60 s). The samples were investigated in three replicates (n ¼ 3), each measurement was done using freshly prepared sponge substrate. Furthermore, before the determination utilizing spongebased sampling all ISEs were checked in their beaker-based analytical performance. Thus only ISEs with Nernstian response were investigated in the sponge-based microfluidic sampling. The calibration of each ISEs was carried out in the concentration range of 104.0-101.0 M of primary ions standard solutions (KCl for KþISEs, NaCl for Naþ-ISEs and KCl for Cl-ISEs) utilizing sponge-based sampling. Similarly as in the pH measurements, the sponge substrates were put onto the unprocessed sample to wick the liquid, while leaving major solids behind. Then the potentiometric cell was pressed with gravitational force against the sponge substrate and the potential of the electrode was registered for 60 s. The samples were investigated in three replicates (n ¼ 3), each measurement was done using freshly prepared sponge substrate. Additionally, the total concentration of potassium and sodium in the wastewater sludge and sweat samples were investigated by PerkinElmer Optima 8300 ICPOES (USA). The chloride concentration was measured by ThermoFisher ICS110 ion chromatography (IC) (USA). Before the ICPOES and IC were used, samples were filtered out by Acrodisc 25 mm Syringe Filter with 0.45 mm supor membrane (Pall, USA) and 10 times diluted. The experiments were conducted at room temperature (23e25  C).

analyte that is wicked into the sponge matrix. Furthermore, the pH of sponge substrates after the contact with ultra-pure water (pH ¼ 7.24) was investigated. As a result, all sponges were characterized with close to neutral pH. The PU sponge was found slightly acidic while cellulose sponge was found slightly basic. The slight deviation from pH of cellulose and PU sponges may be caused by their manufacturing processes, e.g. the alkaline additive used and residual reactant present in the final product [25,35]. Taken that the PU sponges absorption liquid capacity was the highest, while the evaporation rate was the lowest, PU based sponge substrates were further considered for solid-liquid separation that would benefit the potentiometric measurements done by ISEs. The FESEM of the cross-section of a PU sponge substrate before (i) and after (ii, iii) microfluidic solution sampling of wastewater sludge is presented in Fig. 2. FESEM was performed at two different zones on the PU sponge substrate, namely (ii) the upper part where the wicked solution from the samples meet the potentiometric cell for ion determination and (iii) the separation part where the PU sponge was in direct contact with the wastewater sludge. For comparison, clean PU sponge was also presented (i). Already by naked eye inspection, there was a clear boundary between the part (ii) and (iii), indicating separation of bigger solids from the solution. PU sponge

3. Results and discussion 3.1. 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 m3) than other cellulose (134 kg m3) and natural (59 kg m3) sponge substrates, thus it was able to retain larger volume of liquid in its structure. Furthermore, the evaporation rate was the lowest for PU sponge substrates, while natural sponge substrates were characterized with highest evaporation rate. Owning to the evaporation induced concentration changes, subsequent error in concentration were found to be 1.8, 8.6 and 14.4% for PU, cellulose and natural sponge substrates, respectively. The highly porous nature of the PU sponge substrates and its ability to wick much more liquid directly translated in a better retention of the liquid within the structure and lower influence of concentration change after undesired solution evaporation. Thus it was concluded that in the context of short solution sampling integrated with 60 s potentiometric measurement time the sponge-based microfluidic sampling pose no significant change in the concentration of the

Fig. 2. The cross-section of a PU sponge substrate before (i) and after (ii, iii) microfluidic solution sampling of wastewater sludge.

Table 1 Physico-chemical characterization of sponge substrates used for microfluidic sampling. Sponges 1

Liquid absorption capacity (mL g Evaporation rate (mL g1 min1) pH

)

PU sponge

Cellulose sponge

Natural sponge

70.2 ± 8.4 1.2 ± 0.3 6.4 ± 0.1

20.2 ± 1.7 1.7 ± 0.9 7.3 ± 0.3

20.1 ± 0.4 2.9 ± 0.8 7.0 ± 0.2

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effectively stop the transport of major solids through its volume, which is assured by highly porous nature of the sponge. FESME of (Fig. 2. (iii)) clearly presents full coverage of sponge walls with wastewater sludge solid matter. Naturally, it is not expected that sponge would stop fine particles to go through to the electrode surface. From FESEM it is clear, as compared to Fig. 2. (i), that fine particles go through the volume of the sponge to the electrode surface (Fig. 2. (ii)). Such particulates however, should not pose thread of damaging the ISM surface, as reported previously ISEs can tolerate solid fine particulates in the solution [16]. Thus the sponge substrate not only allows transport of ions from the sample to the electrode surface for ion analysis but also protects the electrode from mechanical damages caused by often rough nature of mixed solid-liquid samples. 3.2. Heavy metals sorption onto the sponge substrates In previous studies, when microfluidic sampling utilizing paperand textile-based substrates integrated with potentiometric sensors were used, the Super-Nernstian response of ISEs occurred as a result of heavy metal ions depletion at the ISM surface. Such effect was ascribed to the binding of metal ions to the functional groups present in the paper substrates and/or to concurrent adsorption of metals onto the substrate [4,19,20]. In general, it was found that paper substrates uptake heavy metals and that uptake is correlates with the concentration of the heavy metal in the pre-treating solution [4]. Table 2 presents the sorption capacity of heavy metals (Cd and Pb) in all sponge substrates (measured by ICP-OES) after sponge substrates pre-treatment in various concentrations of Cd(NO3)2 and Pb(NO3)2 (106 to 103 M). Furthermore, PU sponge substrate was chosen to visualize the concentration dependent preconditioning of the sponge substrate and its effect on sorption of heavy metals (Cd and Pb), Fig. 3. Similarly as in paper-based microfluidic sampling, heavy metals undergo sorption to sponge substrates. For all sponge substrates, much higher sorption rates of lead than of cadmium were observed. Previously, the Pb2þ also showed much stronger binding affinity to paper substrates than Cd2þ [4]. The ion sorption of heavy metal ions onto sponge substrate may have either chemical or physical origin. For example, the adsorption of the metal ions is affected by the electronegativity and ionic radii of ions [36,37], thus the Pb2þ with greater electronegativity than Cd2þ can easier adsorbed at the sampling substrate. Moreover cellulose and natural sponge substrates were found to uptake significantly more cadmium and lead during the pretreatment conditioning than PU based sponge substrates. These in fact acted as heavy metal accumulators, and could be considered as suitable substrates for removal of heavy metals in the environmental samples, e.g. natural sponge directly or after modification can be used for purification of contaminated water [38,39]. Thus when applying cellulose and natural sponge substrates in

Fig. 3. The sorption capacity of heavy metals (Cd and Pb) in PU sponge substrates determined using ICP-OES after sponge substrates pre-treatment in various concentration of Cd(NO3)2 and Pb(NO3)2.

microfluidic sampling of analytes, the concentration of the sample may be easily altered. On the other hand PU sponge substrates were characterized with the lowest heavy metal sorption capacity, even up to 2e4 times lower than that measured previously for paperbased substrates [4]. In the previous study, in paper-based microfluidic sampling, modification of paper substrates by inorganic salts resulted in diminishing super Nernstian response as a result of binding of metal ions to the functional groups present in the paper substrates and/or to concurrent adsorption of metals onto the sampling substrate [4]. In sponge-based microfluidic sampling, although smaller, the heavy metal ion binding and nonspecific adsorption is still expected. To further diminish nonspecific adsorption, chemical modification of sponge substrates in order to change hydrophilicity/hydrophobicity of the sampling substrate and to inactive binding sites on the substrate, e.g. by application of lipophilic salts such as tridodecylmethylammonium chloride may be needed [4]. To conclude the experiment represents sorption capacity of heavy metal ion during concentration dependent pre-conditioning of a sponge substrates at fixed time of 30 min, which may not directly reflect measuring conditions used in sponge-based microfluidic sampling (measuring time 60 s and different liquid to substrate ratio). Nonetheless it provides useful information of the capacity of which substrates are able to load heavy metals and since PU sponge substrate exhibited the lowest heavy metal sorption capacity, only the PU based sponge substrates were further considered for microfluidic analyte sampling.

Table 2 The sorption capacity of heavy metals (Cd and Pb) in sponge substrates determined using ICP-OES after sponge substrates pre-treatment in various concentrations of Cd(NO3)2 and Pb(NO3)2. Heavy metal ion

Pre-treatment concentration

3

Total element concentration after pre-treatment (ppm) PU sponge

Cellulose Sponge

Natural sponge

Cadmium

10 M 104 M 105 M 106 M

40.9 ± 1.1 34.4 ± 0.1 16.8 ± 0.2 3.0 ± 0.2

1205.9 ± 8.7 484.4 ± 5.2 35.7 ± 0.5 4.1 ± 0.0

25710.5 ± 168.2 21047.1 ± 82.0 3010.4 ± 4.2 356.3 ± 1.1

Lead

103 M 104 M 105 M 106 M

186.5 ± 1.4 115.9 ± 1.3 98.5 ± 1.0 34.9 ± 1.4

3531.0 ± 50.0 1180.6 ± 6.2 224.1 ± 0.4 26.7 ± 0.5

46762.5 ± 151.3 34109.8 ± 146.3 5336.5 ± 13.5 300.9 ± 3.3

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Fig. 4. Potentiometric cell response using the Kþ-ISEs (left) and Naþ-ISEs (right) in 107.0-101.0 M KCl and NaCl, respectively utilizing beaker-based and sponge-based microfluidic sampling measuring setups.

3.3. Sponge based microfluidic solution sampling coupled with Kþ-, Naþ, and Cl-, Cd2þ-, Pb2þ-ISEs The potentiometric cell response in beaker- and sponge-based microfluidic sampling integrated with Kþ-ISEs and Naþ-ISEs is shown in Fig. 4. In general, the dynamic response of the spongebased measurements was characterized with larger potential noise than beaker-based measurements, which was especially pronounced at low analyte concentrations. The noise in the potential response, however, subsided after approx. 5e10 s, thus collecting potential after 60 s measurement time was done on already equilibrated potentiometric cell. The slopes of Kþ- and Naþ-ISEs were 55.8 and 59.1 mV dec1 respectively considering the concentration range of 104.0-101.1 M of primary ion in sponge-based sampling. For comparison, using beaker-based sampling, the slopes were 57.0 and 56.4 mV dec1, respectively. For both, Kþ- and NaþISEs the low detection limits (LDL) were found comparable between beaker- and sponge-based microfluidic sampling. The slight lower LDL however for beaker-based Kþ-ISEs was recorded, which maybe be caused by the low reproducibility of the potential at

Fig. 5. Potentiometric cell response using the Cl-ISEs in 107.0-101.0 M KCl utilizing beaker-based and sponge-based microfluidic sampling measuring setups.

lowest analyte concentration [18]. Similar potentiometric response was obtained when utilizing textile-based sampling for both Kþand Naþ-ISEs [21]. The potentiometric cell response in beaker- and sponge-based microfluidic sampling integrated with Cl-ISEs is shown in Fig. 5. The slopes for Cl- ISEs in beaker- and spongebased sampling were 60.7 and 59.2 mV dec1, respectively. The similar response of Cl- ISEs was observed previously when studying paper- and textile-based sampling integrated with chloride-sensitive electrodes [20,21]. In conclusions, the potentiometric response of all cation- and anion-sensitive sensors integrated with sponge-based microfluidic solution sampling was found comparable to beaker-based measuring setup. Thus the newly developed microfluidic solution sampling integrated with ISEs would be viable option for the analysis of clinically relevant ions, e.g. Kþ, Naþ and Cl [21,34]. The potentiometric cell response in beaker- and sponge-based microfluidic solution sampling integrated with Cd2þ-ISEs and Pb2þ-ISEs is shown in Fig. 6. The slopes of the calibration curves for Cd2þ-ISEs and Pb2þ-ISEs considering the concentration range of 105.0-101.3 M Cd2þ and 105.0-102.2 M Pb2þ were 27.2 and 30.0 mV dec1, respectively. Subsequently, for beaker-based potentiometric measurements, the slopes were 27.7 and 28.8 mV dec1, respectively. The Cd2þ-ISEs presented comparable potentiometric response for beaker- and sponge-based sampling (LDL ¼ 105.0 M Cd2þ), while for Pb2þ-ISEs, the sponge-based had tenfold higher lower detection limit than that of beaker-based measurement (106.0 M Pb2þ). Lead has higher sorption capacity than cadmium onto sponge-based substrates (as investigated in previous chapter) thus for Pb2þ-ISEs the difference of the LDL between beaker- and sponge-based sampling potentiometric can be possibly attributed to concentration-dependent sorption dynamics of metals onto the sampling substrate. Furthermore, Cd2þ- and Pb2þ-ISEs showed comparable (no super-Nernstian behavior) potentiometric responses between sponge-based and modified paper-based microfluidic sampling [4]. Contrary to sponge-based microfluidic sampling where no pre-treatment of the substrate is needed, in order to eliminate unfavourable super-Nernstian effect observed when utilizing paper-based microfluidic solution sampling, pre-treatment of paper in the primary ion conditioning solution must be performed [4]. To conclude, the direct use of spongebased substrates in microfluidic sampling in the determination of heavy metals, up to sub micromolar concentration range was found feasible. This also, simplifies the protocol of measurement when microfluidic sampling is used, as previously the measurements of

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Fig. 6. Potentiometric cell response using the Cd2þ-ISEs (left) and Pb2þ-ISEs (right) in 107.0-101.0 M Cd(NO3)2 and Pb(NO3)2, respectively utilizing beaker-based and sponge-based microfluidic sampling measuring setups.

heavy metals were only possible with prior pre-treatment of paper substrates [4].

3.4. Determination of pH, Kþ, Naþ, Cl in environmental and clinical samples The pH glass electrode (flat surface) was integrated with sponge-based microfluidic solution sampling to investigate the measurements of pH in environmental samples containing the low volumes of liquid and the high contents of solids, such as wastewater sludge, wet soil and mud. Before the measurements, the performance of the sponge-based microfluidic sampling integrated with pH electrode was investigated by application of three pH standard buffers, namely pH ¼ 4.01, 7.00 and 9.21. The pH of buffers when utilizing PU sponge-based microfluidic sampling were 4.03 ± 0.01, 6.94 ± 0.9, 9.23 ± 0.05, respectively. That indicates that in buffered solutions, the pH of the sample is unaffected by the physico-chemical nature of the sampling substrate. The pH analysis in environmental samples done by sponge- and beaker-based measuring setups integrated with pH glass electrode is shown in Table 3. Consequently, all measurements of pH done when utilizing sponge-based solution sampling were found slightly higher than that for beaker-based pH analysis. For paper- and textile-based pH measurements, much higher deviation of the pH between beakerand microfluidic sampling measuring setups were observed. The

Table 3 The pH in environmental samples utilizing sponge-based microfluidic sampling.

Wastewater sludge Wet soil Mud

Solution-based

Sponge-based

6.42 ± 0.03 7.15 ± 0.03 5.07 ± 0.03

6.88 ± 0.06 7.42 ± 0.07 5.98 ± 0.06

alteration of the pH measurements were associated with the presence of the side functional groups as well as manufacturing process of both paper and textile substrates [20,21]. Furthermore, the sponge-based microfluidic sampling coupled with ISEs was tested in both environmental and clinical analysis of ions, e.g. Kþ, Naþ and Cl. To validate the sponge-based microfluidic sampling, the total concentration of potassium and sodium in both samples were investigated by ICP-OES and the concentration of chloride was investigated by IC. The concentrations of Kþ, Naþ and Cl in wastewater sludge and sweat samples as determined using Kþ, Naþ, Cl-ISEs, ICP-OES and IC are presented in Table 4. The concentrations of Kþ, Naþ and Cl using ISEs integrated with sponge-based microfluidic solution sampling were found comparable to measurements done by ICP-OES and IC. For wastewater sludge sample ion determination, the deviation between measurements done by ISEs and ICPMS/IC were found to be 6, 5 and 4% for Kþ, Naþ and Cl, respectively. For sweat sample A, the deviation were 3, 5 and 7% for Kþ, Naþ and Cl, respectively. While for sweat sample B, the deviation were 5, 3 and 10% for Kþ, Naþ and Cl, respectively. The discrepancy of potentiometric determination and ICP-MS/IC may be caused by cumulative error caused by different ions speciation and their availability in environmental samples, error caused by evaporation of the solvent (1.8% as indicated before) in microfluidic sampling and lack of an adjustment of ionic strength of the standard solutions for analysis done with ISEs. Despite that, the sponge-based microfluidic determination of ions was deemed comparable to ICP-MS/IC, which indicates that sponge-based microfluidic solution sampling integrated with ISEs is a feasible measuring setup in environmental and clinical analysis of ions. 4. Conclusion This work opens new area of material use for microfluidic solution sampling, e.g. application of polyurethane sponge substrates.

Table 4 The concentrations of Kþ, Naþ and Cl in wastewater sludge and sweat samples as determined using Kþ-, Naþ-, Cl-ISEs coupled with sponge-based microfluidic sampling, ICPOES and IC (n ¼ 3).

Wastewater sludge Sweat A Sweat B

log cþ K Kþ -ISE

log cK ICP-OES

log cþ Na Naþ -ISE

log cNa ICP-OES

log ce Cl Cl¡ -ISE

log ce Cl IC

3.28 ± 0.03 2.24 ± 0.03 2.12 ± 0.01

3.08 ± 0.004 2.31 ± 0.001 2.02 ± 0.003

2.13 ± 0.10 1.23 ± 0.02 1.18 ± 0.05

2.40 ± 0.004 1.29 ± 0.002 1.22 ± 0.002

2.39 ± 0.03 1.29 ± 0.04 1.38 ± 0.03

2.48 ± 0.004 1.21 ± 0.003 1.25 ± 0.001

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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 of solids. Thus, the sponge substrate not only allows transport of ions from the sample to the electrode surface for ion analysis but also protects the electrode from mechanical damages. It was found that spongebased microfluidic solution sampling integrated with ISEs was found compatible with ion-selective electrodes sensitive to Kþ, Naþ, Cd2þ, Pb2þ, Cl and pH. Furthermore owning to low heavy metal sorption capacity, out of three investigated sponge types, only polyurethane based sponge was found suitable to be used as substrate for microfluidic solution sampling. Contrary to previous findings on paper-based microfluidic solution sampling, the application of sponge-based microfluidic sampling allowed measurements of heavy metals without prior modification of the sampling substrate. Finally, the determination of potassium, sodium and chloride in wastewater sludge and sweat samples done with sponge-based microfluidic solution sampling integrated with ISEs was found similar to analysis done by ICP-OES and IC.

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

Declaration of competing interest

[19]

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.

[20]

Acknowledgements The authors would like to acknowledge the Economic Development Board (Singapore) for financial support of this research. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.aca.2019.09.024. References [1] R. Parsons, Fundamentals of electrochemical analysis, J. Electroanal. Chem. (1994), https://doi.org/10.1016/0022-0728(94)03416-8. [2] R.E. Sturgeon, S.S. Berman, A. Desaulniers, D.S. Russell, Pre-concentration of trace metals from sea-water for determination by graphite-furnace atomicabsorption spectrometry, Talanta (1980), https://doi.org/10.1016/00399140(80)80023-3. [3] E.P. Achterberg, C. Braungardt, Stripping voltammetry for the determination of trace metal speciation and in-situ measurements of trace metal distributions in marine waters, Anal. Chim. Acta (1999), https://doi.org/10.1016/ S0003-2670(99)00619-4. [4] R. Ding, V. Krikstolaityte, G. Lisak, Inorganic salt modified paper substrates utilized in paper-based microfluidic sampling for potentiometric determination of heavy metals, Sens. Actuators B Chem. (2019), https://doi.org/10.1016/ j.snb.2019.03.079. [5] A.M. Massadeh, A.A. Alomary, S. Mir, F.A. Momani, H.I. Haddad, Y.A. Hadad, Analysis of Zn, Cd, As, Cu, Pb, and Fe in snails as bioindicators and soil samples near traffic road by ICP-OES, Environ. Sci. Pollut. Res. (2016), https://doi.org/ 10.1007/s11356-016-6499-2. [6] K. Sreenivasa Rao, T. Balaji, T. Prasada Rao, Y. Babu, G.R.K. Naidu, Determination of iron, cobalt, nickel, manganese, zinc, copper, cadmium and lead in human hair by inductively coupled plasma-atomic emission spectrometry, Spectrochim. Acta Part B At. Spectrosc. (2002), https://doi.org/10.1016/S05848547(02)00045-9. [7] M. Mende, D. Schwarz, C. Steinbach, R. Boldt, S. Schwarz, Simultaneous adsorption of heavy metal ions and anions from aqueous solutions on chitosandinvestigated by spectrophotometry and SEM-EDX analysis, Colloid. Surf. Physicochem. Eng. Asp. (2016), https://doi.org/10.1016/j.colsurfa.2016.08.033. [8] G. Baumbach, T. Limburg, J.W. Einax, Quantitative determination of sulfur by high-resolution graphite furnace molecular absorption spectrometry, Microchem. J. (2013), https://doi.org/10.1016/j.microc.2012.08.011. [9] T. Wang, Inductively coupled plasma optical emission spectrometry, in: Anal.

[21]

[22]

[23]

[24] [25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34] [35]

[36]

Instrum. Handbook, second ed., 2010, https://doi.org/10.1201/ 9780849390395.ch3. M.R. Ely, B.R. Ely, T.D. Chinevere, C.P. Lacher, H.C. Lukaski, S.N. Cheuvront, Evaluation of the Megaduct sweat collector for mineral analysis, Physiol. Meas. (2012), https://doi.org/10.1088/0967-3334/33/3/385. T. Blaz, B. Ba
s, J. Kupis, J. Migdalski, A. Lewenstam, Multielectrode potentiometry in a one-drop sample, Electrochem. Commun. (2013), https://doi.org/ 10.1016/j.elecom.2013.05.021. R. De Marco, G. Clarke, B. Pejcic, Ion-selective electrode potentiometry in environmental analysis, Electroanalysis (2007), https://doi.org/10.1002/ elan.200703916. E. Bakker, P. Bühlmann, E. Pretsch, Carrier-based ion-selective electrodes and bulk optodes. 1. General characteristics, Chem. Rev. (1997), https://doi.org/ 10.1021/cr940394a. J. Bobacka, A. Ivaska, A. Lewenstam, Potentiometric ion sensors based on conducting polymers, in: Electroanalysis, 2003, https://doi.org/10.1002/ elan.200390042.
ski, G. Lisak, J. Kupis, A. Jasin
ski, M. Bochen
ska, Lead(II)-selective M. Guzin ionophores for ion-selective electrodes: a review, Anal. Chim. Acta (2013), https://doi.org/10.1016/j.aca.2013.04.044. G. Lisak, F. Ciepiela, J. Bobacka, T. Sokalski, L. Harju, A. Lewenstam, Determination of lead(II) in groundwater using solid-state lead(II) selective electrodes by tuned galvanostatic polarization, Electroanalysis (2013), https://doi.org/ 10.1002/elan.201200330. G. Lisak, J. Bobacka, A. Lewenstam, Recovery of nanomolar detection limit of solid-contact lead (II)-selective electrodes by electrode conditioning, J. Solid State Electrochem. (2012), https://doi.org/10.1007/s10008-012-1725-4. G. Lisak, A. Ivaska, A. Lewenstam, J. Bobacka, Multicalibrational procedure for more reliable analyses of ions at low analyte concentrations, Electrochim. Acta (2014), https://doi.org/10.1016/j.electacta.2014.02.091. J. Cui, G. Lisak, S. Strzalkowska, J. Bobacka, Potentiometric sensing utilizing paper-based microfluidic sampling, Analyst (2014), https://doi.org/10.1039/ c3an02157b. G. Lisak, J. Cui, J. Bobacka, Paper-based microfluidic sampling for potentiometric determination of ions, Sens. Actuators B Chem. (2015), https://doi.org/ 10.1016/j.snb.2014.07.044. G. Lisak, T. Arnebrant, T. Ruzgas, J. Bobacka, Textile-based sampling for potentiometric determination of ions, Anal. Chim. Acta (2015), https://doi.org/ 10.1016/j.aca.2015.03.045. J. Ding, N. He, G. Lisak, W. Qin, J. Bobacka, Paper-based microfluidic sampling and separation of analytes for potentiometric ion sensing, Sens. Actuators B Chem. (2017), https://doi.org/10.1016/j.snb.2016.11.128. Y. Luo, W. Guo, H.H. Ngo, L.D. Nghiem, F.I. Hai, J. Kang, S. Xia, Z. Zhang, W.E. Price, Removal and fate of micropollutants in a sponge-based moving bed bioreactor, Bioresour. Technol. (2014), https://doi.org/10.1016/ j.biortech.2014.02.107. whatispolyestercom, What Is Polyester j what Is Polyester, Polyester, 2017. M. M€ artson, J. Viljanto, T. Hurme, P. Laippala, P. Saukko, Is cellulose sponge degradable or stable as implantation material? An in vivo subcutaneous study in the rat, Biomaterials (1999), https://doi.org/10.1016/S0142-9612(99) 00094-0. V.O.A. Tanobe, T.H.S. Flores-Sahagun, S.C. Amico, G.I.B. Muniz, K.G. Satyanarayana, Sponge gourd (luffa cylindrica) reinforced polyester composites: preparation and properties, Def. Sci. J. (2014), https://doi.org/ 10.14429/dsj.64.7327. N.T. Cervin, C. Aulin, P.T. Larsson, L. Wågberg, Ultra porous nanocellulose aerogels as separation medium for mixtures of oil/water liquids, Cellulose (2012), https://doi.org/10.1007/s10570-011-9629-5. H. Peng, H. Wang, J. Wu, G. Meng, Y. Wang, Y. Shi, Z. Liu, X. Guo, Preparation of superhydrophobic magnetic cellulose sponge for removing oil from water, Ind. Eng. Chem. Res. (2016), https://doi.org/10.1021/acs.iecr.5b03862. C. Su, H. Yang, H. Zhao, Y. Liu, R. Chen, Recyclable and biodegradable superhydrophobic and superoleophilic chitosan sponge for the effective removal of oily pollutants from water, Chem. Eng. J. (2017), https://doi.org/10.1016/ j.cej.2017.07.157. A. Siddiqa, A. Shahid, R. Gill, Silica decorated CNTs sponge for selective removal of toxic contaminants and oil spills from water, J. Environ. Chem. Eng. (2015), https://doi.org/10.1016/j.jece.2015.02.026. D. Wu, Z. Yu, W. Wu, L. Fang, H. Zhu, Continuous oil-water separation with surface modified sponge for cleanup of oil spills, RSC Adv. (2014), https:// doi.org/10.1039/c4ra07583h. J. Bobacka, Potential stability of all-solid-state ion-selective electrodes using conducting polymers as ion-to-electron transducers, Anal. Chem. (1999), https://doi.org/10.1021/ac990497z. N.K. Joon, N. He, M. Wagner, M. C
ardenas, J. Bobacka, G. Lisak, Influence of phosphate buffer and proteins on the potentiometric response of a polymeric membrane-based solid-contact Pb(II) ion-selective electrode, Electrochim. Acta (2017), https://doi.org/10.1016/j.electacta.2017.08.126. S. Fuggle, J. Munro, Clinical Biochemistry Reference Ranges Handbook, vol. 8, 2018.
ska, A. Prociak, The influence of rapeseed oil-based polyols on the M. Kuran foaming process of rigid polyurethane foams, Ind. Crops Prod. (2016), https:// doi.org/10.1016/j.indcrop.2016.05.016. M.R. Mehrasbi, Z. Farahmandkia, B. Taghibeigloo, A. Taromi, Adsorption of lead and cadmium from aqueous solution by using almond shells, Water, Air,

R. Ding, G. Lisak / Analytica Chimica Acta 1091 (2019) 103e111 Soil Pollut. (2009), https://doi.org/10.1007/s11270-008-9883-9. [37] T. Bohli, Comparative study of bivalent cationic metals adsorption Pb(II), Cd(II), Ni(II) and Cu(II) on olive stones chemically activated carbon, J. Chem. Eng. Process Technol. (2013), https://doi.org/10.4172/2157-7048.1000158. [38] Z. Liu, B. Li, Y. Pan, K. Shi, W. Wang, C. Peng, Z. Wang, X. Ji, Adsorption

111

behavior of hydrophilic Luffa sponges to heavy metal ions in water system, Chem. J. Chin. Univ. (2017), https://doi.org/10.7503/cjcu20160760. [39] A. Adewuyi, F.V. Pereira, Underutilized Luffa cylindrica sponge: a local bioadsorbent for the removal of Pb(II) pollutant from water system, Beni-Suef Univ. J. Basic Appl. Sci. (2017), https://doi.org/10.1016/j.bjbas.2017.02.001.