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Wearable chemical sensors have seen significant development in the last several years, owing to the abundance of (bio)chemical information which can be extracted from sweat and other body fluids by non-invasive means. Wireless transmission of analytical data is of paramount importance in such wearable sensor systems, since it ensures maximum mobility of the user [1]. pH of perspiration is an important analyte for both medical diagnostics and exercise monitoring and it also affects the concentration of other biomarkers in sweat [2]. We present here a miniature pH sensor for optical monitoring of sweat pH. The sensor is constructed by integrating a pH-sensitive textile with a wireless platform. The pH sensitive textile is itself constructed by first covalently immobilising a pH indicator dye, GJM- 492 (pKa on cellulose layers: 6.1 ; suitable for covering the entire range of sweat pH) [3], onto cellulose particles, dispersing the particles within a dissolved hydrogel, and applying this suspension to a polymer shirt. This immobilisation procedure enables modification of different types of textiles and increases stability of the sensor. Decrease in the intensity of reflected light from an LED source (absorbed by GJM-492) is detected by a photodiode on the wireless platform. The fully automated miniature wireless platform records the pH data and transfers it wirelessly via radio-frequency identification (RFID) or near field communication (NFC) to a remote readout station or smartphone.

Further development of conductometric sensor for organic vapours based on organic semiconductive thin polymer films was undertaken. Previous searches showed that this kind of sensors has a great potential. The sensor response in respect to the ethanol concentration was shown to be highly linear on all types of electrodes [1] (4 point - P, 4 track - T, Quad - Q, Dual - D). All the electrodes had the junction resistance higher than polymer resistance. P electrode has shown to be the most problematic and unstable, and had the biggest fraction of junction resistance in total resistance. Q type electrode has shown to be the most sensitive one, and had the lowest absolute values of resistance, i.e. the highest conductivity. T and D electrodes had the best repeatability, good sensitivity and behaved similarly. Experimental setup remained the same, as presented before. To investigate how number of spin-coated Poly(3, 4-ethylenedioxythiophene)-poly(styrenesulfonate) or PEDOT-PSS films influences the behaviour of the sensor, electrodes with one and three layers of film were compared. Electrodes with three layers of polymer film were shown to be more stable and with better repeatability. However, for stabilization of sensor response longer period of time was needed. All the electrodes with three layers had about 10% smaller fraction of junction resistance in total resistance. In order to find the bottleneck of the process dynamic responses of sensors were recorded. With air stirring, the sensors response achieved stabilization two to four times quicker compared without air stirring. Exposing the sensor directly to ethanol vapours, without evaporation process, few times quicker response stabilization resulted, compared to evaporation process with air stirring. From these experiments it can be concluded that mass transfer trough boundary layer between liquid ethanol and bulk of air is the bottleneck of the process. This was confirmed with a mathematical model, too.

The availability of analogue signal inputs on radio-frequency identification circuits is driving new opportunities for contactless sensing. We are developing applications outside of typical cool- chain temperature tracking by designing novel sensors onto custom RFID labels compatible with the ISO15693 smartcard protocol. This poster describes a contactless microenvironment sensor developed for the long-term in-situ and non- destructive monitoring of steel reinforced concrete (RC) structures. The sensor consists of a custom four-point electrode-array interfaced to a high-frequency (HF) RFID tag. The four-point electrode measures the local resistivity of the concrete and a separate on-chip sensor measures the temperature in the concrete microenvironment. The microenvironment sensor could be embedded in concrete cover layers or other bulk aggregate materials such as cement, mortar, sand or soil to a depth of around 50 cm when operated in either passive (batteryless) or semi-passive modes. Multiple sensors may be distributed throughout a structure to cover larger surface areas or for depth profiling. Data on the temperature and resistivity of the microenvironment in the immediate surrounds of each embedded sensor is then gathered with an RFID reader. This data is subsequently correlated to the probable corrosion status of the steel reinforcement bars (rebars), and may be used to estimate when a threshold or limit state might be reached.

Passive chemiresistor sensors with integral wireless power and data links are in development, and their potential applicability for the detection of organic vapors is demonstrated. The chemiresistors comprise a planar gold electrode array onto which chemically sensitive thin films are cast using micro-fabrication techniques. The thin film sensors are interfaced to a proprietary radio-frequency smartcard designed for use as a distributed chemical or biological detector, and which is compatible with the ISO15693 radio-frequency identification (RFID) wireless protocol. The measurement of thin film sensor resistance is performed by a four-contact method, and is controlled with on-card electronics. Thin polymer films based on the organic semiconductors 3, 4-ethylenedioxythiophene-poly(styrenesulfonate) (PEDOT-PSS) and polyaniline (PANI) have been used as chemically sensitive thin films to demonstrate function of this platform for detection of gaseous analytes. Potential applications of smartcard chemiresistors include mobile and wearable sensors for chemical exposure and contamination monitoring and smart personal diagnostics. The standardization of short-range wireless radio protocols such as Bluetooth, RuBee, ZigBee, WiFi and RFID is creating new markets for distributed sensors and sensor networks, and the fusion of chemical and biosensor technologies with short-range, low-cost, wireless technologies will create new opportunities for chemical and biological sensor systems in healthcare, environmental monitoring, process and quality control, and chemical and biological threat detection [1]. We are developing high-value applications beyond typical cool-chain temperature tracking by designing novel sensors onto custom RFID labels compatible with the ISO15693 smartcard protocol. The RFID protocol is particularly suitable for short-range contactless sensor applications, since it supports both passive (batteryless) and semi-passive (powered) devices and has an anti-collision protocol that supports reading of up to 255 sensors in a single scan. This work is concerned with developing and integrating conductometric chemiresistor sensors onto a short-range wireless platform with which to address a subset of these emerging applications. RFID-based sensor chips are now commercially available for temperature logging [4], and investigational chemical devices have been reported for both gases [5] and vapors [6]. We have developed a custom RFID processor and antenna onto a credit-card sized device [fig.1] which includes a conductometric sensor interface [fig.2]. The smartcard interfaces with a low-cost disposable part on which the chemiresistor is fabricated [fig.3]. First results demonstrate a reversible response of the sensors on exposure to ethanol vapour [fig.4]. The smartcard chemiresistors described here could in future be suitable for use in distributed wireless data applications and chemical sensor networks where organic vapours are to be detected.

Ovaj rad izrađen je kao dio znanstvenog projekta kojemu je krajnji cilj razvijanje bežičnog senzora za nedestruktivno in-situ praćenje korozije armiranobetonskih konstrukcija. Rad povezuje nekoliko naoko veoma različitih područja: korozijska ispitivanja, armirani beton i primjenu bežičnih senzora. Cilj rada je bio ispitati ponašanje grubodisperznih sustava, te istražiti mogućnosti mjerenja električne provodnosti/otpora i temperature primjenom metoda koje koriste izmjeničnu (AC) struju i metoda koje koriste pulsnu istosmjernu struju (pulsed DC). Senzor je izveden kao četveropolna elektroda, koja se sastoji od četiri vijka od nehrđajućeg čelika pričvršćena na karticu koja radi na principu radio-frekvencijske identifikacije (RFID). Dizajnirani su i razvijeni različiti prototipi 4-polnih elektroda i pripadajuće RFID kartice. Očitavanja podataka provedena su bežično, pomoću RFID antene, koja pobuđuje senzor da mjeri provodnost/otpor i temperaturu, te prikuplja podatke. Čitav je spektar parametara koji daju vrijedne informacije o korozijskom procesu, a moguće ih je pratiti pojedinačno ili u kombinaciji, što daje točniju i potpuniju sliku stanja konstrukcije. Iz mnoštva mjerenja koja su na raspolaganju, za ovaj rad je izabrano mjerenje električne provodnosti/otpora i temperature. Izbor je bio takav zbog dostupnosti aparature za spomenuta mjerenja, te zbog jednostavnosti eksperimenata. Procijenjeno je da mjerenja provodnosti/otpora i temperature daju pouzdanu informaciju o tome razvija li se korozijski proces ili ne, kao i je li okruženje u koje želimo smjestiti konstrukciju agresivno ili nekorozivno. Pijesak između ostalog sadrži i neke topljive soli koje bi mogle utjecati na mjerenja provodnosti suspenzija. Stoga je prije svega provedena karakterizacija modelnog sustava, da se eliminira njihov utjecaj. Pijesak je ispiran destiliranom vodom i mjereni su pH i provodnost filtrata na konduktometru pomoću konduktometrijske ćelije. Mjerenja provodnosti provedena su metodama koje koriste izmjeničnu struju, a mjerenja otpora metodama koje koriste pulsnu istosmjernu struju. Za mjerenja s izmjeničnom strujom upotrijebljen je konduktometar, dok su osciloskopom provedena mjerenja s pulsnom istosmjernom strujom. Provodnost i otpor mjereni su u otopinama natrijevog klorida širokog raspona koncentracija, od 1 M do 5.3 M. Ista su mjerenja provedena i u suspenzijama pijeska u otopinama NaCl. Iz mjerenja konduktometrom, uporabom standardne elektrode za mjerenje provodnosti, izračunate su geometrijske konstante prototipnih četveropolnih elektroda (engl. cell constant, CC). Poznavanjem geometrijske konstante elektrode moguće je otpore otopina NaCl, dobivene mjerenjem s RFID karticom preračunati u provodnosti za svaku pojedinu elektrodu, te usporediti mjerenja s konduktometrom i mjerenja s RFID karticom. Provedena su također i mjerenja provodnosti i otpora sa zlatnim elektrodama u otopinama NaCl u rasponu koncentracija 0.01 do 3 mM. Ova su mjerenja poslužila kao metoda za uspostavljanje veze između mjerenja konduktometrom i mjerenja RFID tehnologijom.

Sol-gel glasses have been widely used as solid matrices for immobilisation of chemical and biochemical reagents used in chemical sensors and biosensors, particularly in optical sensors [1]. Sol-gel technology is especially convenient for development of optical sensors since it allows low-temperature preparation of optically transparent porous thin films with tunable properties. In addition, preparation of sol-gel sensing layers in a form of thin films using spin-coating offers a simple and reproducible deposition method. Processability of sol-gel thin films using standard deposition methods as well as their versatility as immobilisation matrices for different reagents including biomolecules such as proteins, enzymes and antibodies, makes them ideal candidates for development of chemically sensitive layers in integrated chemical sensors and biosensors. Especially attractive is the possibility of using sol-gel thin films as a part of planar waveguides in integrated optoelectronic sensing devices. A new generation of chemical sensors and biosensors is emerging as a result of the integration of chemical and biosensor technologies with wireless devices, and has particularly attractive potential applications in healthcare, personal care, environmental monitoring, process and quality control, and chemical and biological threat detection [2]. We have developed an OptiTag as a wireless chemical sensor platform based on the ISO standard radio-frequency identification (RFID) protocol, shown in Figure 1. OptiTag is the fusion of an optoelectronic detector with an RFID processor into a credit-card sized device and has potential application as a chemical sensor node in a wireless sensor network (WSN). Fig. 1 OptiTag with LEDs and a photodiode, and the development electronic board The objective of the work presented here is to develop and integrate an optical chemical sensor and optoelectronic interface onto a wireless RFID tag and to demonstrate its function via optical determination of pH and urea as model analytes. For this purpose, pH sensitive thin-films were made on glass substrates by spin-coating a cocktail of the colour indicator dye bromocresol green (BCG) together with a sol-gel precursor and surfactant. Immobilised bromocresol green (BCG) indicator dye reversibly changes colour in the visible region of the spectrum from pale yellow at pH 4 (protonated state) to dark blue at pH 8 (de-protonated state) with the response in the range of seconds or less. The absorbtion spectrum has an isosbestic point at a wavelength of 505 nm and a strong absorbtion peak at 615 nm which increases with pH. Coated glass chips were placed onto the optoelectronic interface of the OptiTag consisting of a pair of light emitting diodes (LEDs) operating at wavelengths of 505 nm (cyan, λ iso) and 617 nm (red-orange, λ sig). They are used to sequentially illuminate the pH sensitive BCG thin-films and the transmission of light through the film at both wavelengths is determined with the photodiode before and after exposure to buffer solutions of different pH values. Next, urea sensitive sol-gel thin films were developed by immobilising urease enzyme. The sensing scheme is based on the enzymatic degradation of urea which generates OH- ions causing an increase in pH value, which may be monitored using the pH sensor described above. For this application, urease was immobilised in sol-gel thin films using different thin film configurations (single-layer or multi-layer configurations). Optimisation of the sol-gel cocktail composition and the spin-coating processing parameters for different film thicknesses, were peformed. Also, the effects of different pH buffers on enzyme activity were studed and the results are presented. It has been demonstrated that by adapting and modifying the specific chemistry and optoelectronic detector measurement strategy, the OptiTag platform could be deployed in various chemical and biological sensing applications. Such applications might in the future include wearable sensors for chemical and biological exposure and contamination monitoring, environmental monitoring, and long-term in-situ monitoring as embedded sensors. The research presented here is supported by Erasmus Technology LLP, UK, the National Foundation for Science, Higher Education and Technological Development of the Republic of Croatia (NZZ), and the MZOS RH (grant number: 125-0000000-3221). [1]Jerónimo, P. C. A, Araújo, A. N., Conceicao, M., Montenegro, B. S. M.: Talanta 72 (2007) 13 [2]Byrne, R., Diamond D., Nature Materials 5(6) (2006) 421