Volatile Organic Compounds Sensing Using Optical Fibre Long Period Grating with Mesoporous Nano-Scale Coating

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Hromadka, Jiri and Korposh, Sergiy and Partridge, Matthew and James, Stephen and Davis, Frank and Charlton, Alex and Crump, Derrick and Tatam, Ralph (2017) Volatile organic compounds sensing with use of fibre optic sensor with long period grating and mesoporous nano-scale coating. Sensors, 17 (2). 205/1- 2015/16. ISSN 1424-8220

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Article Volatile Organic Compounds Sensing Using Optical Fibre Long Period Grating with Mesoporous Nano-Scale Coating

Jiri Hromadka 1,2, Sergiy Korposh 1,3,*, Matthew Partridge 3, Stephen W. James 3, Frank Davis 4, Derrick Crump 5 and Ralph P. Tatam 3

1 Optics and Photonics Group, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK; [email protected] 2 Institute for Environmental Studies, Charles University in Prague, Benatska 2, Prague 2 CZ-128 01, Czech Republic 3 Centre for Engineering Photonics, Cranfield University, Cranfield, Bedfordshire MK43 0AL, UK; m.c.partridge@cranfield.ac.uk (M.P.); s.w.james@cranfield.ac.uk (S.W.J.); r.p.tatam@cranfield.ac.uk (R.P.T.) 4 Department of Engineering and Applied Design, University of Chichester Bognor Regis, West Sussex PO21 1HR, UK; [email protected] 5 Environmental Science and Technology Department, Cranfield University, Cranfield, Bedfordshire MK43 0AL, UK; d.crump@cranfield.ac.uk * Correspondence: [email protected]; Tel.: +44-115-7484-701

Academic Editors: Eduard Llobet and Stella Vallejos Received: 24 November 2016; Accepted: 17 January 2017; Published: 8 February 2017

Abstract: A long period grating (LPG) modified with a mesoporous film infused with a calixarene as a functional compound was employed for the detection of individual volatile organic compounds (VOCs) and their mixtures. The mesoporous film consisted of an inorganic part, SiO2 nanoparticles (NPs), along with an organic moiety of poly(allylamine hydrochloride) polycation PAH, which was finally infused with the functional compound, p-sulphanato calix[4]arene (CA[4]) or p-sulphanato calix[8]arene (CA[8]). The LPG sensor was designed to operate at the phase matching turning point to provide the highest sensitivity. The sensing mechanism is based on the measurement of the refractive index (RI) change induced by a complex of the VOCs with calixarene. The LPG, modified with a coating of 5 cycles of (SiO2 NPs/PAH) and infused with CA[4] or CA[8], was exposed to chloroform, benzene, toluene and acetone vapours. The British Standards test of the VOCs emissions from material (BS EN ISO 16000-9:2006) was used to test the LPG sensor performance.

Keywords: long period grating (LPG); volatile organic compounds (VOCs); phase matching turning point (PMTP); mesoporous ﬁlm; layer-by-layer (LbL); p-sulphanato calix[8]arene (CA[8]); p-sulphanato calix[4]arene (CA[4])

1. Introduction Most people spend more than 90% of their time indoors [1], in daily contact with household products, such as paints and paint strippers, solvents, sprays and wood preservatives, disinfection products, repellents and air fresheners, all emitting or containing volatile organic compounds (VOCs). VOCs consist of a broad mixture of gases and cause various short-term and long-term (delayed) adverse health effects, with some proven as animal or human carcinogens. They are also determined as one of the possible causes of sick building syndrome (SBS) [2]. The indoor concentration of VOCs is higher in comparison to outdoor levels [2]. The mean total VOCs (TVOCs) concentration was measured to be 0.2–0.4 mg·m−3 during a survey of British households [3].

Sensors 2017, 17, 205; doi:10.3390/s17020205 www.mdpi.com/journal/sensors Sensors 2017, 17, 205 2 of 16

There are guidelines for individual substances (e.g., formaldehyde, benzene) set by the World Health Organization (WHO) for homes [4], while the occupational limits are set by the Occupational Safety and Health Administration (OSHA) in the US [5] and by the Health and Safety Executive (HSE) in UK [6]. Scientific evidence shows that the irritation of the occupants is associated with indoor concentration of TVOCs in the range 0.2 to 5 mg·m−3 [3], while no effect seems to be observed at concentrations below 0.2 mg·m−3 [7]. As material leakage represents the most significant indoor source of VOCs (50%, the remainder is associated with human activities), the overall acceptable emission rate of the materials should be kept below 0.03 mg·m−2·h−2 (the approximation considers the size of the room, possible leakage surface area and ventilation rate) [2]. The legal framework concerning the testing of VOCs leakage from paints is covered by EU 2004/42/CE, which sets the limits for VOCs emissions that occur due to use of organic solvents in certain paints, varnishes and vehicle refinishing products. The guidelines for material testing are also covered by ISO standards, where following are the most appropriate EN ISO 16000-9—Indoor air—Part 9—Determination of the emission of volatile organic compounds from building products and furnishing—Emission test chamber method and EN ISO 16000-10 Determination of the emission of volatile organic compounds from building products and furnishing—Emission test cell method). Passive flux sampling, emission test cells (ETC) and field and laboratory emission cells (FLEC) have been considered to be useful for material leakage measurements to determine the level of VOCs [8]. Gas chromatography-mass spectroscopy (GC-MS) were used for VOC analysis in this case [8]. Passive sampling does not require an air pump and an air-flow meter, but very volatile agents can be lost because of back-diffusion [9]. Passive samplers provide average concentration data for an exposure period that is often days or weeks to achieve appropriate sensitivity. Passive badges use charcoal or other media as an adsorbent. The badge is left in the environment during the sampling period (commonly from 8 h to 1 week) and then is sent to the lab for the further analysis [10]. When measurement of the concentration dynamic (changes of exposure in time) is required, an active (pumped) approach is applied [11]. Real time measurement is undertaken using photo-ionization detectors (PIDs), but the accuracy depends on the VOCs present in the actual mixture [10]. While longer time averages may be more relevant when possible health effects of the pollutant are considered, in the case of chronic effects the concentration dynamic is important [9]. The most precise and also the most expensive approach is active sorption/chemical analysis. The sampled air is flowed by the pumps into the tubes which include the sorbent of organic polymer resins (e.g., Tenax) or activated charcoal. The sorbent traps the present VOCs and GC-MS is then used for VOCs analysis. The VOCs are measured individually and then the TVOCs is subsequently calculated [10]. ETCs and FLECs can be small and portable or can be large scale, with volumes of up to 80,000 dm3. The chambers are usually fabricated from glass or stainless steel, because there should be no reaction between the surface of the chamber and the analyte of interest. The condition inside the cells can be controlled and FLECs have the advantage that can be used outside the lab. The air flows through the cell to the sorbent tubes and the trapped VOCs are further analysed by GC-MS [8]. While large-scale ETCs allow the testing of complete, intact equipment, the experiment takes a long time and is highly expensive. The costs are smaller for FLECs, with passive flux samplers being less expensive still [8]. Application of the inexpensive and portable sensors with sufficient sensitivity that can provide results in real time to measure leakage of VOCs is highly desirable and will benefit both industry and consumers. Fibre optic-based TVOCs sensors could potentially provide real time measurements, sufficient accuracy and low cost. Other advantages include their small size, they can work even in extreme conditions, enable remote monitoring with no electrical power needed at the sensing point and in addition they are immune to electromagnetic interference [12]. Sensors 2017, 17, 205 3 of 16

A variety of fibre optic based chemical sensing platforms have been reported and investigated during the last decade [13,14]. Among them, fibre optic long period gratings (LPGs) modified with appropriate functional materials have been considered to provide high sensitivity and selectivity towards target compounds [15–22]. An LPG consists of a periodic perturbation of the refractive index of the optical fibre core, which couples fundamental core mode into the co-propagating cladding modes. This coupling is manifested in the transmission spectrum of the optical fibre as a series of loss, or resonance bands. Each of them corresponds to coupling to a different cladding mode and each shows a different sensitivity to environmental perturbation [23]. The wavelengths at which the core mode is coupled to cladding modes are determined by the phase matching Equation (1): λx = ncore − nclad(x) Λ (1) where λx represents the wavelength at which light is coupled to the LP0x cladding mode, ncore is the effective refractive index of the mode propagating in the core of the fibre, nclad(x) is the effective index of the LP0x cladding mode and Λ is the period of the LPG [23]. The sensitivity of LPGs to external perturbations such as strain, temperature or refractive index (RI) change relies on the measurement of the shifts of the central wavelengths of the resonance bands that are proportional to the magnitude of the measurand [23]. For the development of fibre optic chemical and bio-sensors, the surface of the optical fibre is modified with a functional material that can selectively interact with the target compound. The selection of the material determines the analytes to which the coated LPG is sensitive and the thickness influences the sensitivity [24]. The response of the transmission spectrum of the LPG to the nanoscale coating deposition is characterized by a shift of the central wavelengths of the attenuation bands, which are caused by the change of the effective index of the cladding modes, and this change is optical-thickness dependent [25]. In addition, the highest sensitivity and the best performance are obtained when the LPG operates at or near the phase matching turning point (PMTP) [26], requiring an appropriate combination of grating period and optical thickness of the coating. (The optical thickness is derived from the geometrical thickness of the coating multiplied by the refractive index of the coating.) Fibre optic sensors containing LPGs with functional coating have been used to measuring relative humidity [20], ammonia [15,27] or VOCs [28–30], and can also be used for a range of other materials, e.g., in bio-sensing [19,22,31]. Various methods have been employed for the deposition of the functional film onto the optical fibre [15,21,30–33]. Among these methods, the layer by layer (LbL) technique (also known as electrostatic self-assembly (ESA) technique), based on the deposition of the oppositely charged materials, allows control of film thickness with molecular precision. Using an LbL approach, the entire coating can be made from the sensitive material, which reacts with the compound of interest, leading to a change of its refractive index. Another option is that the sensitive material is infused into a mesoporous coating [15,16]. The sensitive material should fulfil the following criteria: have appropriate morphology, porosity, flexibility and thickness for the sensing selectivity, and matrix porosity for the speed of analyte diffusion [34]. A range of functional coatings—calixarene [28,29], zeolites [35] or diphenylsiloxane and titanium cross-linker have been used to modify LPG based fibre optic sensors to measure VOCs [36]. In this work, a single mode optical fibre containing a long period grating (LPG) was modified with a mesoporous thin film that was infused with calixarene molecules that acted as the functional compound for VOCs detection. Calixarene molecules contain a number of phenol or resorcinol aromatic rings connected together to a larger ring and the whole molecule looks like a bowl [37]. The analyte of the interest reacts with calixarene and becomes temporarily entrapped, however only weak interactions occur (no covalent bond is created) which enables easy liberation from the cavity and this effect causes the reversibility of the sensor. The sensitivity of the reaction depends on the size and shape of the molecule of interest and for this reason the specific reactions to different VOCs may be expected [30]. Sensors 2017, 17, 205 4 of 16

The use of calixarenes for the detection of VOCs has been reported previously. Ca[8] was shown Sensors 2017, 17, 205 4 of 16 to be sensitive to chloroform [38], benzene and toluene [30] and the responses of CA[4] exposure to chloroformThe LPG and was benzene exposed [29 to] and concentrations toluene [39] up have to beenthe saturated explored. vapour concentrations of acetone, benzene,The chloroform LPG was exposed and toluene. to concentrations The influence up of to two the different saturated sizes vapour of calixarene concentrations molecules of acetone, on the performancebenzene, chloroform of the VOCs and toluene. sensor was The influenceinvestigated of two and different a comparison sizes of of calixarene their sensitivities molecules was on undertakenthe performance. The sensor of the was VOCs also sensor used to was measure investigated mixtures and of aVOCs comparison leaking offrom their two sensitivities different types was ofundertaken paints, with. The aim sensor of assessing was also its used performance to measure in mixtures real life of applications. VOCs leaking In fromaddition, two different the chemical types compositionof paints, with of the aim VOCs of assessing was monitored its performance by GC-MS. in real life applications. In addition, the chemical compositionThe experiment of the VOCs performed was monitored simulated by GC-MS. the conditions stated in the previously mentioned BritishThe—ISO experiment standards. performed simulated the conditions stated in the previously mentioned British—ISO standards. 2. Materials and Methods 2. Materials and Methods 2.1. Materials 2.1. Materials PAH (Mw: 75,000), NaOH 1 M aqueous solution, NH3, benzene, toluene, chloroform and acetone PAH (Mw: 75,000), NaOH 1 M aqueous solution, NH3, benzene, toluene, chloroform and were purchased from Sigma-Aldrich. SiO2 NPs (SNOWTEX 20L) was purchased from Nissan acetone were purchased from Sigma-Aldrich. SiO NPs (SNOWTEX 20L) was synthesised Nissan Chemical. Calix-[8]-arene (CA[8]) and calix-[4]-arene2 (CA[4]) 1 mM solution was synthetized in Chemical. p-sulphanato calix[8]arene (CA[8]) and p-sulphanato calix[4]arene (CA[4]) 1 mM solution house [40]. All of the chemicals were of analytical grade reagents and were used without further was synthetized in house [40]. All of the chemicals were of analytical grade reagents and were used purification. Deionized water (18.3 MΩ·cm) was obtained by reverse osmosis followed by ion without further purification. Deionized water (18.3 MΩ·cm) was obtained by reverse osmosis followed exchange and filtration (Millipore, Direct-QTM, Nottingham, UK). by ion exchange and filtration (Millipore, Direct-QTM, Nottingham, UK).

2.2.2.2. Sensor Sensor Fabrication Fabrication AnAn LPG LPG of of length length 40 40 mm mm with with a a grating grating period period of of 110.9 110.9 μmµm was was fabricated fabricated in in boron boron-germanium-germanium coco-doped-doped optical optical fibre fibre (Fibercore (Fibercore PS750 PS750,, Fibercore, Fibercore, Southampton, Southampton, UK UK)) with with a cut a cut-off-off wavelength wavelength of 670of 670nm in nm a point in a- point-by-pointby-point fashion, fashion, side-illuminating side-illuminating the optical the fibre optical by the fibre output by the from output a frequency from- a quadrupledfrequency-quadrupled Nd:YAG laser, Nd:YAG operating laser, at operating 266 nm. at 266 nm. The selected grating period facilitates coupling to a higher order cladding mode (LP019) at the The selected grating period facilitates coupling to a higher order cladding mode (LP019) at the PMTP,PMTP, where where its its response response to to external external refractive refractive index index is is largest largest [41]. [41]. The The use use of of optical optical fibre fibre with with a a shortshort cut cut-off-off wavelength wavelength ensures ensures that that resonance resonance bands bands a att their their PMTP PMTP lie lie towards towards the the red red end end of of the the visiblevisible spectrum, spectrum, allowing allowing the the transmission transmission spectrum spectrum of the of LPG the to LPG be monitored to be monitored using an inexpensive using an inexpensivelight source, light halogen source, lamp, halogen and a lamp, low-cost and CCD a low spectrometer.-cost CCD spectrometer. TheThe transmission transmission spectrum spectrum of of the the optical optical fibre fibre was was recorded recorded by by coupling coupling the the output output from from a a tungstentungsten-halogen-halogen lamp lamp (Ocean (Ocean Optics Optics HL HL-2000,-2000, Ocean Ocean Optics, Optics, Oxford, Oxford, UK UK)) into into the the fibre, fibre, analysing analysing thethe transmitted transmitted light light using using a a fibre fibre coupled coupled CCD CCD spectrometer spectrometer (Ocean (Ocean Optics Optics HR4000 HR4000,, Ocean Ocean Optics, Optics, Oxford,Oxford, UK UK).). The The grati gratingng period period was was selected selected such such that that the the LPG LPG operated operated at at or or near near the the phase phase matching turning point, which, for coupling to a particular cladding mode (in this case LP019), ensured matching turning point, which, for coupling to a particular cladding mode (in this case LP019), ensured optimizedoptimized sensitivity. sensitivity. TheThe schematic schematic illustration illustration of of the the LPG LPG sensor sensor is is shown shown in in F Figureigure 11..

FigureFigure 1 1.. SchematicSchematic illustration illustration of of the the long long period period grating grating ( (LPG)LPG) sensor. sensor.

2.3. Thin Film Deposition

A mesoporous thin film of SiO2 NPs was deposited onto the LPG using an electrostatic self- assembly approach as previously described [15,16,27]. An LPG was fixed in a Teflon holder constructed with a compartment to accommodate a solution. Briefly, the region of the optical fibre with the LPG was rinsed with deionized water and immersed in a 1 wt % NaOH ethanol and water (3:2, v/v) solution for 20 min, leading to a negatively charged surface. The optical fibre was then sequentially immersed into a solution containing a

Sensors 2017, 17, 205 5 of 16

2.3. Thin Film Deposition

A mesoporous thin film of SiO2 NPs was deposited onto the LPG using an electrostatic self-assembly approach as previously described [15,16,27]. An LPG was fixed in a Teflon holder constructed with a compartment to accommodate a solution. Briefly, the region of the optical fibre with the LPG was rinsed with deionized water and immersed in a 1 wt % NaOH ethanol and water (3:2, v/v) solution for 20 min, leading to a negatively charged surface. The optical fibre was then sequentially immersed into a solution containing a positively charged polymer, PAH, and a solution containing negatively charged SiO2 NPs for 20 min each, resulting in the alternate deposition of PAH and SiO2 NPs layers on the surface of the fibre. The fibre was rinsed with distilled water, and dried by flushing with nitrogen gas after each deposition step. The immersion in the PAH and SiO2 nanoparticles was repeated until the 5 layers were deposited. The transmission spectrum (TS) of the LPG was measured after each deposition. As was previously shown, functional compounds can be infused into mesoporous thin films [15] to provide sensor with its specificity to the desired analyte. CA[8] and CA[4] were used as the functional materials to provide the sensor reported here with the sensitivity to VOCs. In general, the thickness and porosity of the sensitive element affect the performance of coated LPG sensors [25,41]. It was previously shown that the optimal sensitivity to external refractive index was achieved [25] when 10 layers of SiO2 NPs thin film were deposited onto the surface of the LPG sensors. In this work, however, since the functional compound is infused into the mesoporous thin film, the optical thickness, the product of geometrical thickness and refractive index, changes, hence altering the optimal conditions of the LPG sensor. Here, 5 layers were found to provide the optimal performance of the LPG sensor in terms of sensitivity and response time. The film porosity affects the refractive index of the film. The mesoporous structure created by silica nanoparticles enables the infusion of the sensitive element—calixarene. The porosity between the nanoparticles affects the amount of possible binding sites for the calixarene-VOC complexes and thus indicates the VOCs sensitivity. The film porosity affects the refractive index of the film, in this case allowing the deposition of a coating of refractive index 1.2 [15]. The deposition of a well-developed mesoporous structure of SiO2 nanoparticles film with a mean pore radius of 12.5 nm and specific surface area of 50 m2·g−1 onto an LPG was reported previously [15,42]. The mesoporous structure created by silica nanoparticles enables the infusion of the sensitive element—calixarene, which also acts to raise the refractive index of the coating. Calixarenes form a natural cavity with an approximate diameter of around 1–2 nm. These small cavities allow condensation of the target vapour below the natural condensation point of the vapour [39]. The pore dimensions influence the number of possible binding sites for the calixarene-VOC complexes and thus affect the sensitivity to VOCs. The LPG modified with the (PAH/SiO2)5 coating was immersed into a 1 mM of CA[8] aqueous solution for 2 h followed by washing and drying. This LPG sensor is referred as CA[8]. After all experiments (VOCs measurements) were completed with CA[8] sensor, the LPG was immersed into NH3 solution for 2 h to remove the CA[8] molecules. The LPG was then dried and immersed into CA[4] solution (the procedure was the same as for CA[8]). LPG modified with CA[4] is referred as the CA[4] sensor. The transmission spectra of the LPG during CA[8] and CA[4] infusion were measured, along with monitoring the intensity change at a particular wavelength located on the edge of one the resonance bands. The process was terminated when the changes in the central wavelengths of the resonance bands and in the intensity were saturated.

2.4. Volatile Organic Compounds Measurement The LPG modiﬁed with the CA[8] and CA[4] was tested for VOCs sensitivity in the laboratory and during a paint leakage test simulation as follows. Sensors 2017, 17, 205 6 of 16

The LPG was ﬁxed in a closed cell and the solution of the VOC of interest (benzene, toluene, acetone and chloroform), of volume of 10, 30, 50, 100 µL (according to the saturation level), was injected in the cell (standard Petri dish with volume of 100 cm3) and then was left until complete evaporation. The cell was opened shortly before adding each volume. The LPG was kept straight and taut to avoid any bending which can affect its performance. The relation between the injected volume and the VOC concentration in the Petri dish for each compound tested is shown in Table1. Sensors 2017, 17, 205 6 of 16

Table 1. Relation between the injected volume and the concentration of the volatile organic Table 1. Relation between the injected volume and the concentration of the volatile organic compound compound (VOC). (VOC).

VOCVOC AcetoneAcetone BenzeneBenzene Chloroform Toluene VolumeVolume (µL) (µL) ConcentrationConcentration (ppm) (ppm) 1010 26,01226,012 19,80619,806 23,67923,679 17,97817,978 3030 78,03878,038 59,42059,420 71,03771,037 n/an/a 5050 130,064130,064 99,03499,034 118,396118,396 n/an/a 100 260,128 n/a n/a n/a 100 260,128 n/a n/a n/a

The temperature temperature and and relative relative humidity humidity (RH) (RH) were simultaneously were simultaneously recorded recorded as both are as interfering both are interferingfactors (the factors LPG itself (the is LPG sensitive itself to is temperature, sensitive to temperature, while SiO2 NPs while thin SiO ﬁlm2 NPs coating thin causes film coating the sensitivity causes theto RH). sensitivity A ‘One to Wire’ RH). sensor A ‘One with Wire’ precision sensor of with 0.5 ◦precisionC and 0.6% of was0.5 °C used and to 0.6 control% was temperature used to control and temperatureRH during the and all experiments RH during the(iButton all experiments® Hygrochron (iButton Temperature/Humidity® Hygrochron Temperature/Humidity Logger, part number Logger,DS1923, part from number Maxim DS1923, Integrated fromTM, Maxim High Wycombe, Integrated UK).TM, High Wycombe, UK). The scheme used to assess the sensitivity of the LPG to VOCs is shown inin FigureFigure2 2..

Figure 2.2. Scheme for VOCs sensitivity experiments.

2.2.5.5. Materials Leakage Test The simulation of of the the ISO ISO standard standard test test involved involved exposure exposure of of the the LPG LPG sensor sensor to tothe the mixture mixture of VOCsof VOCs emitted emitted from from paints paints within within a a chamber chamber consisting consisting of of a a desiccator desiccator base base and and a a stainless steel emission cell,cell, asas shownshown inin FigureFigure3 .3. The The LPG LPG was was ﬁxed fixed in in a glassa glass desiccator desiccator (with (with estimated estimated volume volume of 2 of7.5 7.5 L) forL) 4.5for h;4.5 a pieceh; a piece of cardboard of cardboard of surface of sur areaface 100 area cm 2100, freshly cm , paintedfreshly bypainted commercially by commercially available paintsavailable sold paints for domestic sold for use,domestic was placed use, was inside placed the desiccator.inside the desiccator. The paint source The paint was source Plasti-kote was projectPlasti- paint,kote project 143S Antique paint, 143S gold Antique (further gold referred (further as spray referred paint) as andspray Wickes paint) Master and Wickes universal Master multi-surface universal primermulti-surface (can paint). primer (can paint). TS were were recorded recorded every every 1 min 1 duringmin during all experiments. all experiments. The central The wavelengths central wavelengths of the attenuation of the attenuationbands were bands obtained were using obtained Spectrum using InterrogationSpectrum Interrogation Routine software Routine [software43] and Origin [43] and was Origin used was for usedfurther for data further analysis. data Theanalysis. temperature The tempera and relativeture and humidity relative werehumidity kept were at the kept same at level the same during level the duringexperiments the experiments and simultaneously and simultaneously measured using measured OneWire using device. OneWire device. A constant constant airﬂow airflow was was maintained maintained during during the whole the whole experiment. experiment. The incoming The incoming air was controlled air was controlledto be a 1:1 mixtureto be a 1:1 of drymixture and humidof dry airand (initial humid RH air in (initial the cell RH was in measuredthe cell was around measured 60%). around An emission 60%). Ancell, emission a stainless cell, steel a enclosure stainless steel designed enclosure for the designed testing of for emissions the testing of VOCs of emissions from materials, of VOCs (FLEC from®, ® materials,Chematec, (FLEC Roskilde,, Chematec Denmark), Roskilde, was placed Denmark on the top) was of theplaced desiccator on the base. top of Air the provided desiccator to the base. device Air prentersovided the to enclosure the device from enters the the perimeter enclosure and from exits the centrally perimeter where and thereexits iscentrally the facility where to samplethere is the facility to sample the air to determine the presence of VOCs. The FLEC® is further described in the international standard ISO 16000-10, which concerns determination of the emission of VOCs from construction products.

Sensors 2017, 17, 205 7 of 16 air to determine the presence of VOCs. The FLEC® is further described in the international standard SensorsISO 16000-10, 2017, 17, 205 which concerns determination of the emission of VOCs from construction products.7 of 16

FigureFigure 3 3.. TheThe photograph photograph of of measurement measurement set set-up-up used used for for the the paint paint leakage leakage test test [28]. [28].

The air was constantly leaving the measuring system through an air sampling tube on the outlet. The air was constantly leaving the measuring system through an air sampling tube on the outlet. Sorbent tubes (packed with Tenax TA, Sigma-Aldrich, Dorset, UK) were used to collect the mixture Sorbent tubes (packed with Tenax TA, Sigma-Aldrich, Dorset, UK) were used to collect the mixture of VOCs in the air. The air flow rate was 100 mL per minute and sample was collected for 2.5, 5 and of VOCs in the air. The air flow rate was 100 mL per minute and sample was collected for 2.5, 5 and 10 min with the final total volumes of 0.25, 0.5 and 1 L. The samples of volume 0.25 L were used as 10 min with the final total volumes of 0.25, 0.5 and 1 L. The samples of volume 0.25 L were used follows. as follows. The air samples leaking from both paints were evaluated with the use of TD/GC/MS (thermal The air samples leaking from both paints were evaluated with the use of TD/GC/MS (thermal desorption/gas chromatography/mass spectrometry). The VOCs were identified according to peak desorption/gas chromatography/mass spectrometry). The VOCs were identified according to area and expected retention time according to a NIST-library (version 2.0, 25 June 2008) with the peak area and expected retention time according to a NIST-library (version 2.0, 25 June 2008) similarity coefficient ca. 980 (maximum 1000) [44]. The thermal desorption technique was used to with the similarity coefficient ca. 980 (maximum 1000) [44]. The thermal desorption technique desorb VOCs from the tubes and a DB5 gas chromatography column and a mass quadropole was used to desorb VOCs from the tubes and a DB5 gas chromatography column and a mass spectrometer was used for the VOCs analysis. D8-toluene (C6D5CD3) (IS), retention time (RT) = 15.48 quadropole spectrometer was used for the VOCs analysis. D8-toluene (C6D5CD3) (IS), retention min was used as internal standard for GC-MS analysis. Its solution in methanol was injected to all time (RT) = 15.48 min was used as internal standard for GC-MS analysis. Its solution in methanol tubes prior to analysis. Details of this type of analysis method can be found in ISO 16000-6 [45]. was injected to all tubes prior to analysis. Details of this type of analysis method can be found in 3.ISO Results 16000-6 [45]. 3. Results 3.1. Deposition of the Sensitive Film 3.1. DepositionThe transmission of the Sensitive spectrum Film of an unmodified LPG with period of 110.9 µm is shown in Figure 4. AttenuationThe transmission bands in the spectrum region of of 800 an and unmodified 850 nm correspond LPG with period to the LP of019 110.9 claddingµm is mode shown and in Figureoperate4. at the PMTP while the other ones (region of 625 and 670 nm) corresponds to lower cladding modes Attenuation bands in the region of 800 and 850 nm correspond to the LP019 cladding mode and operate andat the are PMTP less sensitive while the for other this onesLPG (regionperiod. LPG of 625-R andshowed 670 nm)higher corresponds sensitivity to than lower LPG cladding-L and for modes this reasonand are was less used sensitive for the for further this LPG analysis. period. LPG-R showed higher sensitivity than LPG-L and for this reasonThe was cladding used for modes the further associated analysis. with the attenuation bands have been calculated through numericalThe cladding modelling modes using associated the phase with matching the attenuation equitation and bands weakly have guided been calculated approximation through to determinenumerical the modelling effective using refractive the phase indices matching of the core equitation and cladding and weakly modes guidedand the approximationknown values of to thedetermine central thewavelengths effective refractive and the grating indices period of the core[46].and cladding modes and the known values of the central wavelengths and the grating period [46].

Sensors 2017,, 17,, 205205 8 of 16 Sensors 2017, 17, 205 8 of 16 LPG 110.9 µm LPG 110.9 µm 100 100

80 LP LP -R 018 019 80 LP LP -R LP 018 019 017 LP -L LP 019 017 LP -L 019 Transmission(%) 60

Transmission(%) 60 CW difference ( CW at LP ) 019 CW difference ( CW at LP ) 700 800 900 019 1000 700 Wavelength800 (nm)900 1000 Wavelength (nm)

Figure 4. Transmission spectrum of an uncoated LPG with period of 110.9 µm measured in air, with FigureFigure 4 4.. Transmission spectrum of an uncoated LPG with period of 110.9 µmµ measured in air, with attenuationTransmission bands at 670 spectrum nm, 800 ofand an 850 uncoated nm corresp LPG withonding period to the of LP 110.9018 andm LP measured019 cladding in air, modes with attenuationattenuation bands bands at at 670 nm, 800 and 850850 nmnm correspondingcorresponding toto thethe LPLP018 and LPLP019 claddingcladding modes modes respectively (the dual resonance was observed for the LP019 cladding 018mode denoted019 as LP019-L (800 respectivelyrespectively (the(the dualdual resonance resonance was was observed observed for for the the LP LP019cladding cladding mode mode denoted denoted as LP as LP-L019 (800-L (800 nm) nm) and LP019-R (850 nm)); CW, central wavelength. 019 019 nm)and and LP019 LP-R019 (850-R (850 nm)); nm)); CW, CW, central central wavelength. wavelength.

The LPG was coated with 5 layers of SiO2 nanoparticles and the changes in transmission spectrum The LPG was coated with 5 layers of SiO2 nanoparticles and the changes in transmission spectrum wereThe monitored LPG was in coatedair and with in solution, 5 layers ofFigure SiO2 5ananoparticles,b respectively. and According the changes to in the transmission results published spectrum in were[15], whichmonitored monitored followed in in air air the and and same in in solution, fabrication solution, Figure Figure procedure, 5a5a,b,b respectively. respectively. the thickness According According of the coating to to the the is results 250 results nm. published published in [15],in [15 which], which followed followed the thesame same fabrication fabrication procedure, procedure, the thickness the thickness of the of coating the coating is 250 is nm. 250 nm.

100 100 100 100 100 100 100 90 100 90 90 80 bare LPG 90 SiO (1) 80 2 90 bare LPG 90 80 SiO (2) SiO 2 (1) 80 2 SiO (3) SiO 2 (2) 80 2 SiO (4) 70 80 Transmission (%) SiO 2 (3) 2 80 SiO (5) Transmission (%) SiO (1) SiO 2 (4) 70 80 2 Transmission (%) 2

60 Transmission (%) SiO (2) SiO (5) SiO 2 (1) 670 675 2 2 60 665 670 SiO (3) SiO 2 (2) 60 2 700 670 675 800 900 SiO (4) 60 700665 670 800 900 SiO 2 (3) 2 700 800 900 SiO (5) Wavelength (nm) 700 Wavelength800 (nm) 900 SiO 2 (4) 2 SiO (5) Wavelength (nm) Wavelength (nm) 2 (a) (b) (a) (b) Figure 5.5. TransmissionTransmission spectraspectra of of LPG LPG with with period period of 110.9of 110.9µm: µm: (a) taken(a) taken in air in beforeair before (black (black) and) afterand Figure 5. Transmission spectra of LPG with period of 110.9 µm: (a) taken in air before (black) and theafter deposition the deposition of 1st (red of 1st), 2nd (red (blue), 2nd), 3rd (blue (green), 3rd), 4th (green (magenta)), 4th and (magenta) 5th layer and of silica 5th layer nanoparticles of silica after the deposition of 1st (red), 2nd (blue), 3rd (green), 4th (magenta) and 5th layer of silica (cyan)nanoparticles and (b) (cyan) taken inand solution (b) taken after in the solution deposition after ofthe 1st deposition (black), 2nd of 1st (red (black),), 3rd (blue 2nd ),(red 4th), ( green3rd (blue) and), nanoparticles (cyan) and (b) taken in solution after the deposition of 1st (black), 2nd (red), 3rd (blue), 5th4th layer(green of) and silica 5th nanoparticles. layer of silica nanoparticles. 4th (green) and 5th layer of silica nanoparticles. In order to endow the sensor with sensitivity to VOCs, the LPG was initially modified by InIn order order to to endow endow the the sensor sensor with with sensitivity sensitivity to VOCs, to VOCs, the LPG the was LPG initially was initially modified modified by applying by applying 5 layers of SiO2 NPs. The observed change in the transmission spectrum is shown in Figure 5 layers of SiO2 NPs. The observed change in the transmission spectrum is shown in Figure5 and it is applying5 and it is5 layers in a good of SiO agreement2 NPs. The withobserved previous change reports in the [15 transmission,25]. Comparison spectrum of theis shown TS of in the Figure LPG in a good agreement with previous reports [15,25]. Comparison of the TS of the LPG measured after its 5measured and it is after in a its good immersion agreement into with CA[8] previous and CA[4] reports solutions [15,25 is]. shown Comparison in Figure of the6a. The TS ofinfusion the LPG of measuredimmersion after into its CA[8] immersion and CA[4] into solutions CA[8] and is shown CA[4] in solutions Figure6a. is Theshown infusion in Figure of CA[4] 6a. The caused infusion a larger of CA[4] caused a larger shift of the central wavelengths (ΔCW at LP019 = 17.96 nm) than CA[8], where shift of the central wavelengths (∆CW at LP019 = 17.96 nm) than CA[8], where only a small change CA[4]only a caused small change a larger was shift observed of the central (2.75 wavelengthsnm). This effect (ΔCW can at be LP explained019 = 17.96 by nm) the than size CA[8], of calixarene where onlywas observeda small change (2.75 nm). was Thisobserved effect (2.75 can be nm). explained This effect by the can size be ofexplained calixarene by molecules. the size of The calixarene smaller molecules. The smaller CA[4] was deposited more effectively in to the pores of the SiO2 mesoporous CA[4] was deposited more effectively in to the pores of the SiO2 mesoporous thin film, while larger molecules.thin film, while The smaller larger CA[8] CA[4] couldn’t was deposited penetrate more effectively. effectively in to the pores of the SiO2 mesoporous CA[8] couldn’t penetrate effectively. thin film,The evolutionwhile larger of theCA[8] TS ofcouldn’t the LPG penetrate when immersed effectively. in the CA[4] solution is shown in Figure 6b. The evolution of the TS of the LPG when immersed in the CA[4] solution is shown in Figure6b. The largestThe evolution shift of ofthe the central TS of wavelength the LPG when as well immersed as the absolutein the CA[4] transmittance solution is changeshown atin theFigure central 6b. The largest shift of the central wavelength as well as the absolute transmittance change at the central Thewavelength largest shift were of observed the central in wavelengththe first 15 min as well and as small the absoluteor no changes transmittance were observed change afterat the an central hour. wavelength were were observed observed in in the the first first 15 15 min min and and small small or or no no changes changes were were observed observed after after an an hour. hour. These results suggest that infusion of the CA[4] into the SiO2 mesoporous thin film saturates after 60 These results suggest that infusion of the CA[4] into the SiO2 mesoporous thin film saturates after Thesemin. results suggest that infusion of the CA[4] into the SiO2 mesoporous thin film saturates after 60 min.60 min.

SensorsSensorsSensors 20172017 2017, ,1717, ,,2 17 20505, 205 9 9of of 916 16of 16

110 110 100 100 100 100 begin begin 15min 90 90 15min 90 LPG-L 90 30min LPG-L LPG-LLPG-L 30min LPG-RLPG-R (SiO ) 45min (SiO ) 2 5 45min 2 5 60min 80 80 LPG-R +CA[8] 60min CA[4]

LPG-R +CA[8] Transmission % / CA[4] Transmission % / 75min

Transmission % / 80 Transmission % / 75min +CA[4] +CA[4] 80 sol sol 90min 90min 70 70 700 700 800 800 900 900 10001000 700 800 900 1000 Wavelength / nm 700 800 900 1000 Wavelength / nm Wavelength / nm Wavelength / nm (a) (a) (b) (b)

Figure 6. (a) TS after (SiO2)5 NPs deposition (black) with infused CA[8] (red) or CA[4] (blue) and (b) FigureFigure 6 6.. (a()a TS) TS after after (SiO (SiO2)52 NPs)5 NPs deposition deposition (black) (black) with with infused infused CA[8] CA[8] (red ()red or )CA[4] or CA[4] (blue (blue) and) and(b) TS of LPG at the infusion of CA[8] into (PAH/SiO2)8 film deposited over LPG in solution at different TS(b )of TS LPG of LPG at the at theinfusion infusion of CA[8] of CA[8] into into (PAH/SiO (PAH/SiO2)8 2film)8 ﬁlm deposited deposited over over LPG LPG in insolution solution at at different different timetimetime intervals. intervals. intervals.

3.2. VOCs Measurement 3.23.2.. VOCs VOCs Measurement Measurement A measurable response to VOCs was observed for LPG modified with both CA[4] and CA[8] in AA measurable measurable response response to to VOCs VOCs was was observed observed for for LPG LPG modified modiﬁed with with both both CA[4] CA[4] and and CA[8] CA[8] in in less than 30 s (Figure 7), even when the volume of the injected solution was 10 µL, corresponding lessless than than 30 30 s s(Figure (Figure 7),7), even when when the the volu volumeme of the injected solution was was 10 10 µL,µL, corresponding corresponding approximately to vapour concentrations of 23,000 ppm vol. for toluene, 30,000 ppm vol. for approximatelyapproximately to to vapour vapour concentrations concentrations of 23,000 of 23,000 ppm ppm vol. for vol. toluene, for toluene, 30,000 ppm 30,000 vol. ppm for chloroform, vol. for chloroform, 25,400 ppm vol. for benzene and 33,000 ppm vol. for acetone. chloroform,25,400 ppm 25,400 vol. for ppm benzene vol. for and benzene 33,000 ppmand 33,000 vol. for ppm acetone. vol. for acetone. With increasing amount of VOC injected the sensor response saturated at 23,000 ppm vol. for WithWith increasing increasing amount amount of of VOC VOC injected injected the the sensor sensor response response saturated saturated at at 23,000 23,000 ppm ppm vol. vol. for for toluene, 152,000 ppm vol. for chloroform, 125,000 ppm vol. for benzene and 298,000 ppm vol. for toluene,toluene, 152,000 152,000 ppm ppm vol. vol. for for chloroform, chloroform, 125,000 125,000 ppm ppm vol. vol. for for benzene benzene and and 298,000 298,000 ppm ppm vol. vol. for for acetone (these values are close to the saturation levels based on the boiling point). The response of acetoneacetone (these (these values values are close toto thethe saturationsaturation levels levels based based on on the the boiling boiling point). point). The The response response of theof the transmission spectra towards different VOCs are shown in Figures S1 and S2. As an example, thetransmission transmission spectra spectra towards towards different different VOCs VOCs are shown are shown in Figures in Figure S1 ands S1 S2. and As S2. an example,As an example, acetone acetone vapour at the concentration close to the saturation point (~298,000 ppm) induced the change acetonevapour vapour at the concentration at the concentration close to close the saturationto the saturation point (~298,000point (~298,000 ppm) ppm) induced induced the change the change in the in the central wavelengths difference of 2.65 nm for CA[4] sensor. incentral the central wavelengths wavelengths difference difference of 2.65 of nm 2.65 for nm CA[4] for CA[4] sensor. sensor.

100 100 100100

90 90 90 90 air air acetone air

acetone Transmission % / air

Transmission % / Transmission % / at saturation level acetone Transmission % / at saturation level acetone 80 at saturation level 80 80 80 at saturation level 880 890 900 910 780 840 900 880 890 900 910 780 840 900 Wavelength / nm WavelengthWavelength / nm / nm Wavelength / nm

(a) (a) (b) (b) Figure 7. Transmission spectrum of (a) CA[4] sensor and (b) LPG-R in detail in air (black) and FigureFigure 7 7.. TransmissionTransmission spectrum spectrum of of (a ()a CA[4]) CA[4] sensor sensor and and (b) LPG-R(b) LPG in- detailR in detail in air ( inblack air ) ( andblack exposed) and exposed to high concentration of acetone (close to the saturation level) (blue). exposedto high concentrationto high concentration of acetone of acetone (close to (close the saturation to the saturation level) (blue level)). (blue).

TheThe characterization characterization of the of the response response of the of the LPG LPG to VOCs to VOCs showed showed that that the the optical optical fibre fibre can can The characterization of the response of the LPG to VOCs showed that the optical ﬁbre can respond respondrespond to the to thehigh high concentrations concentrations of a of range a range of different of different VOCs. VOCs. The The results results from from the theexposure exposure of the of the to the high concentrations of a range of different VOCs. The results from the exposure of the sensor to sensorsensor to individual to individual VOCs VOCs are aresummarized summarized in Table in Table 2. 2. individual VOCs are summarized in Table2.

Sensors 2017, 17, 2 20505 1010 of 16 16

Table 2. Sensitivity of CA[4] and CA[8] to individual VOCs. Table 2. Sensitivity of CA[4] and CA[8] to individual VOCs. Slope of the Calibration Curve [nm/ppm] TypeSlope of Calixarene of the Calibration CA[4] Curve [nm/ppm]CA[8] Type ofVOC Calixarene Δ CA[4] CW019 Difference CA[8] Acetone 5.9 × 10−5 1.0 × 10−5 VOC ∆ CW019 Difference benzene 4.3 × 10−5 1.8 × 10−5 Acetone 5.9 × 10−5 1.0 × 10−5 −5 −5 chloroformbenzene 4.33.0 ×× 10−5 1.80.7× × 1010−5 chloroformToluene 3.04.7 ×× 10−−55 0.72.0× × 1010−−55 Toluene 4.7 × 10−5 2.0 × 10−5 3.3. Material Leakage 3.3. MaterialThe CA[8] Leakage sensor was exposed to the mixture of VOCs that leaked from the spray paint on the cardboardThe CA[8] in the sensor desiccator. was exposedChanges toin thetransmission mixture of spectrum VOCs that were leaked observed from 5 the min spray after paint the painted on the cardboard inwas the installed. desiccator. The Changes shift of the in transmission central wavelength spectrum continued were observed for a further 5 min 25 after min the and painted then thecardboard changes was saturated. installed. The The LPG shift ofresponded the central immediately wavelength continuedto opening for the a furtherdesiccator 25 min to andremove then the the paintedchanges sample saturated. with The the LPG rapid responded change immediatelyof the central to wavelength opening the observed. desiccator The to removefinal transm the paintedission spectrumsample with was the recorded rapid change three minutes of the central after wavelengththe removal observed.of the painted The finalsample transmission (the desiccator spectrum was shortlywas recorded opened three to remove minutes the after sample the removal and then of theclosed painted again). sample The final (the desiccatorspectrum waswas shortlysimilar openedto that recordedto remove prior the sample to exposure and then to the closed VOCs, again). indicating The final goo spectrumd reversibility was similarof the sensor to that response recorded (F priorigures to S3exposure and S6a). to the VOCs, indicating good reversibility of the sensor response (Figures S3 and S6a). The CA[4] sensor was exposed to to the the same same paint paint (new (new freshly freshly painted painted sample sample was was prepared) prepared) under the same conditions and similar results were obtained in comparison to to the CA[8] sensor (Figure S4). The dynamic response of the CA[4] sensor is shown in Figure8 8a.a. The CA[4] sensor was finally finally exposed to the can paintpaint sample.sample. The transmission spectra are shown in Figure Figures8b 8b and and Figure S5 and S5 and the the dynamic dynamic response response is is shown shown in in Figure Figure S6b. S6b. A A slower initial initial responresponsese of of 0.35 0.35 nm nm (LPG (LPG-R)-R) was was observed observed in in comparison comparison to to 2.22 2.22 nm nm shift shift of of LPG LPG-R-R observed observed after after 5 min5 min after after the the exposure exposure to to the the can can paint paint and and spray spray paint paint sample sample respectively. respectively.

CA[4] sensor - spray paint CA[4] sensor - LPG-R - can paint 4.0 3.5 888.64nm 100 888.88nm 3.0 888.99nm 2.5 890.40nm 2.0 90 890.16nm 1.5

Transmission (%) 1.0 80 5 min CW difference LP Air begin 019 30 min 0.5 120 min Air final

Central wavelength difference (nm) 0.0 0 50 100 150 200 250 300 880 890 900 Wavelength (nm) Time (min) (a) (b)

Figure 88.. ((aa)) Dynamic Dynamic change change of of CA[4] CA[4] sensor sensor during during the the spray spray paint paint experiment experiment and and ( (b)) CA[4] sensorsensor—LPG-R—can—LPG-R—can paint paint experiment experiment—TS—initial—TS—initial one one (black (black), ),final ﬁnal one one (red (red) and) and 5 min 5 min (green (green), 30), min30 min (blue (blue) and) and 120 120 min min (pink (pink) after) after the the paint paint placement. placement.

The experiments showed that the response of CA[4] sensor to the spray paint was larger than The experiments showed that the response of CA[4] sensor to the spray paint was larger than that exhibited by the CA[8] sensor, and that the response of LPG-R was larger than that of LPG-L for that exhibited by the CA[8] sensor, and that the response of LPG-R was larger than that of LPG-L for both sensors. This finding is in agreement with experiments exposing the sensors to individual VOCs. both sensors. This ﬁnding is in agreement with experiments exposing the sensors to individual VOCs. The CA[4] sensor responds differently to the two types of paints, most likely because of their different The CA[4] sensor responds differently to the two types of paints, most likely because of their different VOCs composition. A less pronounced response of LPG CA[4] sensor was observed mainly during VOCs composition. A less pronounced response of LPG CA[4] sensor was observed mainly during the first 30 min during the experiment with the can paint. These observations support the hypothesis the ﬁrst 30 min during the experiment with the can paint. These observations support the hypothesis of the different response to the different VOCs that was found during the experiments exposing the of the different response to the different VOCs that was found during the experiments exposing the

Sensors 2017, 17, 205 11 of 16 sensors to individual VOCs. The central wavelengths of the resonance bands for selected transmission spectra are shown in Table3.

Table 3. Absolute shift of the central wavelengths difference during the paint experiments.

Paint CA[8] Spray CA[4] Spray CA[4] Can

Time/min ∆ CW019 Difference (nm) 0 0 0 0 5 0.96 3.57 0.60 30 3.25 4.53 2.62 120 3.13 4.03 2.63 ﬁnal 0.35 0.12 0.13

The GC-MS analysis showed the different chemical compositions of the two paints used in this work. The results are shown in Figure9. More than 250 different compounds were identified in both paints. This finding indicates that the VOCs with higher molecular weight are present in the can paint. Although it is difficult to obtain an exact list of the chemicals present because the individual peaks overlap, which causes biases in chemical detection, some of the VOCs could be classified. The following VOCs (5 with the highest abundance) were identified for each paint: spray—n-octane, o-xylene, n -nonane, Propan-2-ol, n-decane; can—o-xylene, n-decane, n-nonane, n-undecane, etylbenzene.

(a) (b)

Figure 9. Results from GC-MS (a) spray paint (b) can paint (x-axis represents retention time, y-axis abundance).

4. Discussion The shift in the central wavelength difference induced by the different concentration levels of the tested VOCs is summarized in Figure8. The CA[8] sensor exhibited a smaller response than CA[4] almost in all cases, which is most likely a result of the lower amount of CA[8] infused into the SiO2 thin ﬁlm. The biggest shift was observed for acetone, the lowest for chloroform for CA[4] sensor. No differences were observed between individual VOCs for the CA[8] sensor, Figure 10. The response time was 1 min or less in all experiments, taken when the sensor reached 90% of the total central wavelength shift. It is expected that the sensor will respond to lower concentrations, as the sensor was observed to react to the smallest dose tested. The theoretical limit of detection was stated at 2211 ppm vol. for acetone, 3057 ppm vol. for benzene, 4382 ppm vol. for chloroform and 2743 ppm vol. for toluene. The ∆ CW019 difference of CA[4] sensor was used for the LOD calculation to enable the comparison of the sensor’s sensitivity to individual VOCs. The recalculation had been done for 10 µL injected volume. The concentration

Sensors 2017, 17, 205 12 of 16

Sensorscorresponding 2017, 17, 205 to 10 µL of the analyte was divided by the absolute shift of the central wavelength12 of 16 (in nm) and multiplied by 0.13 (nm) as the resolution of the spectrometer. The standard deviation experimentsacross experiments where where the sensor the sensor was exposedwas exposed to individual to individual VOCs VOCs ranged ranged from from 0.08 0.08 to to 0. 0.2323 nm, nm, suggesting similar values for the experimental and theoretical LOD.

(a) (b)

Figure 10 10.. ((a)) CA[8] and ( b) CA[4] Sensor’s response response related related to to towards selected VOCs at at different concentration levels (the error bars represent the standard deviation).deviation).

The nearly same LOD to acetone, benzene and toluene and 1.5 times larger LOD to chloroform The nearly same LOD to acetone, benzene and toluene and 1.5 times larger LOD to chloroform was obtained. This effect is probably caused by the smaller size of chloroform molecules, which leads was obtained. This effect is probably caused by the smaller size of chloroform molecules, which leads to less interaction between them and the calixarene film. It should be noted that stable experimental to less interaction between them and the calixarene film. It should be noted that stable experimental conditions could not be attained because the chamber was not hermetically sealed and no validated conditions could not be attained because the chamber was not hermetically sealed and no validated measuring technique was used to measure the VOC concentration simultaneously. measuring technique was used to measure the VOC concentration simultaneously. The different responses of the sensors spray paint and to paint in a can could be explained as The different responses of the sensors to spray paint and to paint in a can could be explained follows. There were higher concentrations of aromatic compounds in the spray paint, and higher as follows. There were higher concentrations of aromatic compounds in the spray paint, and higher concentrations of aliphatic compounds in the can paint. This conclusion correlates with the higher concentrations of aliphatic compounds in the can paint. This conclusion correlates with the higher response of the optical fibre sensor to spray paint, because calixarene coated LPGs have previously response of the optical fibre sensor to spray paint, because calixarene coated LPGs have previously shown higher sensitivity to aromatic VOCs that to aliphatic VOCs [30]. shown higher sensitivity to aromatic VOCs than to aliphatic VOCs [30]. No degradation of the functional coating has been observed throughout the experiments, likely No degradation of the functional coating has been observed throughout the experiments, likely a result of the hybrid nature of the sensitive film, which consists of organic and inorganic moieties. a result of the hybrid nature of the sensitive film, which consists of organic and inorganic moieties. Moreover, reversible response was observed due to the weak and reversible chemical interaction Moreover, reversible response was observed due to the weak and reversible chemical interaction between calixarenes and VOCs. between calixarenes and VOCs. The experiments with the paints were undertaken in conditions which are similar to those used The experiments with the paints were undertaken in conditions which are similar to those used for tests which determine the emission of VOCs from building products and furnishing, which are for tests which determine the emission of VOCs from building products and furnishing, which are covered in British standards and international ISO norms. This kind of testing of VOC emission from covered in British standards and international ISO norms. This kind of testing of VOC emission from different materials could be a further target for the application of fibre optic sensors after optimization different materials could be a further target for the application of fibre optic sensors after optimization of the sensor performance. of the sensor performance. The experiments also showed the potential of measuring total or selected VOCs with use of The experiments also showed the potential of measuring total or selected VOCs with use of optical optical fibre technology, however sensitivity at low concentrations (ppb range) has not yet been fibre technology, however sensitivity at low concentrations (ppb range) has not yet been achieved. achieved. The sensitivity achieved to date does not meet the criteria for the 8 h exposure given by Health The sensitivity achieved to date does not meet the criteria for the 8 h exposure given by Health and Safety Executive (HSE), stated as 500, 50, 2 and 1 ppm for acetone, toluene, chloroform and and Safety Executive (HSE), stated as 500, 50, 2 and 1 ppm for acetone, toluene, chloroform and benzene respectively and neither the 15 min exposure limit of 1500 and 100 ppm for acetone and benzene respectively and neither the 15 min exposure limit of 1500 and 100 ppm for acetone and toluene respectively. Further work is required to optimise the sensitivity of the sensor. toluene respectively. Further work is required to optimise the sensitivity of the sensor. 5. Conclusions 5. Conclusions The performance of a fibre-optic long period grating with a nano-scale functional coating of SiO2 NPsThe with performance infused CA[8] of ora fibre CA[4]-optic for VOCslong period response grating has beenwith explored.a nano-scale The functional sensors were coating exposed of SiO to2 NPsindividual with infused VOCs andCA[8] their or mixturesCA[4] for evaporating VOCs response from has the been freshly explored. painted The surfaces. sensors were exposed to individual VOCs and their mixtures evaporating from the freshly painted surfaces. The ability to sense VOCs using functionalized LPGs has been demonstrated, but further work is required to improve the sensitivity, by optimization of the grating period and coating thickness. Based on the experiments presented here, and with improved calibration, it is anticipated that a limit

Sensors 2017, 17, 205 13 of 16

The ability to sense VOCs using functionalized LPGs has been demonstrated, but further work is required to improve the sensitivity, by optimization of the grating period and coating thickness. Based on the experiments presented here, and with improved calibration, it is anticipated that a limit of detection lower than of 100 s of ppm could be achieved. When the response in individual VOCs was investigated, an initial response of the sensor was observed in less than 30 s, and the sensors exhibited difference responses to individual VOCs and mixtures. Two different sizes of calixarene molecules were tested, with CA[4] exhibiting the largest response. Although the sensor cannot discriminate between individual VOCs, a different response was observed to two different commercial paint products, suggesting that the sensor can distinguish between the different mixtures. The different responses can be explained by the presence of lower amounts of VOCs in the can paint and by the smaller response of the sensor to larger molecules. This hypothesis will be tested in future work. The proposed sensor benefits from easy fabrication and low cost fabrication process. The flexibility of the deposition method allows also re-use of the same fibre after cleaning. The response of the sensor was affected by the timescales for the evaporation of the VOC, but was found to be less than 1 min, enabling in situ, repeatable and real time measurement of TVOCs. We can conclude that the LPG reacts to high concentrations of different types of individual VOCs as well as to mixtures evaporating from the freshly painted samples however the more careful control of the conditions during the lab experiments, requiring use of a hermetically sealed chamber with controllable temperature and RH and simultaneous measurement of validated techniques are needed to allow the generation of calibration curves. Optimized operation of the sensor would require the selection of grating period, grating strength and optical thickness of the film such that the resonance band lies at the PMPT, while the thickness of the coating also ensures that the device is operating at the mode transition region [46]. An LPG operating in this was demonstrated in [26]. While there are a number of theoretical treatments of coated LPGs [47], a significant limitation in their use for predicating the optimal parameters lies in the accuracy with which the optical fibre and coating material parameters are known, which can create significant discrepancies in the predictions of the model, particularly for coupling to higher order modes at the PMTP. It has been shown that the influence of environmental interfering parameters, such as temperature, can be corrected by using an array of appropriately configured sensors that exhibit different responses to the analyte and to the interfering parameters [48].

Supplementary Materials: The following are available online at www.mdpi.com/1424-8220/17/2/205/s1, Figure S1: TS of the LPG with infused CA[4] exhibited to acetone (a) LPG-L; (b) LPG-R, benzene; (c) LPG-L; (d) LPG-R, chloroform; (e) LPG-L, f) LPG-R and toluene g) LPG-L, (h) LPG-R, Figure S2: TS of the LPG with infused CA[8] exhibited to acetone (a) LPG-L, benzene (b) LPG-L; (c) LPG-R, chloroform (d) LPG-L; (e) LPG-R and toluene (f) LPG-L; (g) LPG-R, Figure S5: CA[8] sensor—spray paint experiment—TS (a) LPG-L and (b) LPG-R—initial one (black), final one (red) and 5 min (green), 30 min (blue) and 120 min (pink) after the paint placement; (c) dynamic change of the central wavelength during the whole experiment, Figure S6: CA[4] sensor—spray paint experiment—TS (a) LPG-L and (b) LPG-R—initial one (black), final one (red) and 5 min (green), 30 min (blue) and 120 min (pink) after the paint placement, Figure S7: CA[4] sensor—can paint experiment—TS LPG-L—initial one (black), final one (red) and 5 min (green), 30 min (blue) and 120 min (pink) after the paint placement, Figure S8: (a) Dynamic change of a) CA[8] sensor during the spray paint experiment and (b) CA[4] sensor during the can paint experiment—LPG-L (red) and LPG-R (black). Acknowledgments: The authors would like to acknowledge the support of a UK Engineering and Physical Sciences Research Council Platform Grant [No. EP/H02252X/1] and responsive mode grants [No. EP/L010437/1] and [No. EP/N025725/1]. we would also like to acknowledge contribution of Alex Charlton and his help with GS-MS measurements. Author Contributions: S.K. and S.W.J. conceived and designed the experiments. J.H. had conducted all the experimental work and had written the manuscript. M.P. had written the code of the software used for the central wavelength detection. F.D. had fabricated the calixarenes used in all experimental work. A.C. had performed the VOCs analysis by GC-MS device. R.P.T. and D.C. provided their expertise in the field of fibre optic sensing and VOCs detection, respectively. All authors had comments towards the manuscript. Conflicts of Interest: The authors declare no conflict of interest. Sensors 2017, 17, 205 14 of 16

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