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Broad-Range Hydrogel-Based pH Sensor with Capacitive Readout Manufactured on a Flexible Substrate
Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.ORCID iD: 0000-0003-1967-4016
Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.ORCID iD: 0000-0003-2965-0288
Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design. RISE Acreo AB, Bredgatan 33, 601 17 Norrköping, Sweden.ORCID iD: 0000-0002-0631-3804
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2018 (English)In: Chemosensors, ISSN 2227-9040, Vol. 6, no 3, p. 15article id 30Article in journal (Refereed) Published
Abstract [en]

Environmental monitoring of land, water and air, is an area receiving greater attention because of human health and safety concerns. Monitoring the type of pollution and concentration levels is vital, so that appropriate contingency plans can be determined. To effectively monitor the environment, there is a need for new sensors and sensor systems that suits these type of measurements. However, the diversity of sensors suitable for low, battery powered- and large area sensor systems are limited. We have manufactured and characterized a flexible pH sensor using laser processing and blade coating techniques that is able to measure pH between 2.94 and 11.80. The sensor consists of an interdigital capacitance with a pH sensitive hydrogel coating. Thin sensors can reach 95% of their final value value within 3 min, and are stable after 4 min. Good repeatability was achieved in regard to cycling of the sensor with different pH and multiple measurements from dry state. We have also studied the relation between an interdigital capacitance penetration depth and hydrogels expansion. We believe that our passive sensor is suitable to be used in low power and large area sensor networks.

Place, publisher, year, edition, pages
Basel, Switzerland: MDPI, 2018. Vol. 6, no 3, p. 15article id 30
Keywords [en]
interdigital, hydrogel, penetration depth, pH, sensor, coating, thin film, laser ablation, oligo (β-amino esters)
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:miun:diva-34203DOI: 10.3390/chemosensors6030030ISI: 000448395400006Scopus ID: 2-s2.0-85052627188OAI: oai:DiVA.org:miun-34203DiVA, id: diva2:1235378
Available from: 2018-07-25 Created: 2018-07-25 Last updated: 2019-05-09Bibliographically approved
In thesis
1. Hydrogel-based pH-sensors: Development and characterisation of optical and electrical pH sensors based on stimuli-responsive hydrogels
Open this publication in new window or tab >>Hydrogel-based pH-sensors: Development and characterisation of optical and electrical pH sensors based on stimuli-responsive hydrogels
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The ability to measure the chemical parameter pH is of high importance in many areas. With new government regulations and evolving markets, there is a strong motivation for improving such measurements and conducting research on new types of pH sensors and sensor materials. Stimuli-responsive hydrogels (a group of polymers) have attracted a lot of attention in recent decades, due to their ability to be customized to suit many applications. One specific area where they have attracted attention is pH sensor technology.

Two stimuli-responsive hydrogels are used in this thesis. One is a non-toxic hydrogel, 1,4-acryl-terminated oligo(beta-amino esters) (1,4-AOBAE). Although it was previously used in drug- and DNA- delivery systems, it has not (to my knowledge) been used in a sensor configuration, and thus it is interesting to study. The second hydrogel, 1,3-acryl-terminated oligo(beta-amino esters) (1,3-AOBAE), is an improved variant of the first one. This improved hydrogel was synthesized because the original hydrogel crystallizes at room temperature, which meant that it was not optimal for various coating techniques. This hydrogel was characterized and verified for pH responsivity in two sensor configurations: electrical and optical. Designing a hydrogel for a specific application can be a complex procedure due to the many synthesizing parameters. For example, increasing a hydrogel's mechanical strength by introducing a higher degree of cross linking, leads to a smaller mesh size, which in turn leads to a lower diffusion rate and less solution absorption. The two hydrogels examined in this thesis respond to pH changes by absorbing or desorbing water; this change in the hydrogel's water content also changes its effective refractive index and permittivity. These changes can be measured using optical or electrical sensor systems. Three types of sensor systems were used in this thesis to verify the hydrogel's pH response and to ensure that they are suitable for use in thin-film techniques on various substrates (e.g. glass and plastic). The experimental results prove that these hydrogels are suitable for use in both electrical and optical sensor configurations. For electrical systems, a pH range of approximately 3-12 was achieved, and for optical, the range was approximately 2-12. These ranges can likely be improved, as the sensor film delaminated from the substrate at low pHs due to adhesion problems and as measurements above 12 were not conducted.

The findings of this thesis could, after more research, have strong implications for the development of improved pH-sensor configurations, especially for medical and healthcare applications and in environmental monitoring.

Abstract [sv]

Möjligheten att mäta den kemiska parametern pH är av stor betydelse inom många områden. Nya regler från myndigheter och tillväxtmarknader gör att det finns det en stark motivation för förbättring av och forskning om nya typer av pH-sensorer och sensormaterial. Stimuli-responsiva hydrogeler (en grupp polymerer) är ett material som under de senaste årtiondena uppmärksammats på grund av deras förmåga att skräddarsys för att passa många olika applikationer. Ett specifikt område där de har uppmärksammats är i pH-sensorteknik.

I arbetet presenterat i denna avhandling har två stimuli-responsiva hydrogeler använts. Dessa båda icke-toxiska hydrogeler har tidigare använts i olika läkemedel samt till DNA leverans system, men däremot inte till pH sensorer, vilket har gjorts i detta arbete. Hydrogelerna har använts för att konstruera och tillverka optiska pH sensorer baserade på fiberoptik samt elektriska sensorer med kapacitiv utläsning. Att syntetisera en hydrogel för en specifik applikation kan vara komplicerat, vilket beror på de många syntetiseringsparametrar som är inblandade vid tillverkning. Till exempel, leder införande av en högre tvärbindningsgrad till ökad mekaniska styrka hos hydrogelen, men också till en mindre maskstorlek, vilket i sin tur leder till en minskning av diffusions-hastigheten och absorption av lösningen. De två hydrogeler som undersöks i denna avhandling reagerar på pH-förändringar genom att absorbera eller desorbera vatten. Förändring av vattenhalt i hydrogelen ändrar också dess effektiva brytningsindex och permittivitet. Dessa ändringar av brytningsindex eller permittivitet kan med fördel mätas med optiska eller elektriska sensorsystem. Tre olika typer av sensorsystem har använts för att verifiera hydrogelens pH-respons och och dess lämplighet att användas i tunnfilms-teknik på olika substrat (glas och plast). Experimentella undersökningar har visat att de undersökta hydrogelerna är lämpliga att användas i både elektriska och optiska sensor konfigurationer. Det elektriska systemet visade sig känsligt för pH förändringar mellan ~3-12, och optiska system mellan ~2-12. Det observerades att låga pH värden ger en så stor volymsförändring av polymeren att denna då släpper ifrån underlaget. Det är därför troligt att  mätintervallet kan utökas om vidhäftningen mellan polymer och underlag kan förbättras.

Resultaten från denna avhandling kan med ytterligare forskning ha en stark inverkan på utvecklingen av förbättrade pH-sensor konfigurationer, särskilt inom medicinska- och hälsovårds-applikationer samt miljöövervakning.

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden University, 2019. p. 61
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 295
Keywords
Hydrogel, fiber optics, printed electronics, pH, sensor
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-35647 (URN)978-91-88527-93-6 (ISBN)
Public defence
2019-03-19, O102, Holmgatan 10, Sundsvall, 13:15 (English)
Opponent
Supervisors
Available from: 2019-02-19 Created: 2019-02-14 Last updated: 2019-02-20Bibliographically approved

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Hammarling, KristerEngholm, MagnusAndersson, HenrikSandberg, MatsNilsson, Hans-Erik

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