RSC Advances View Article Online PAPER View Journal | View Issue Liquid photonic crystal detection reagent for reliable sensing of Cu2+ in water† Cite this: RSC Adv., 2020, 10,10972 Yixin Zhang and Jianping Ge * A traditional hydrogel photonic crystal sensor is prepared by multiple steps, including colloidal assembly, polymerization, and recognition group modification, and its measurement repeatability is a challenge due to the inevitable deviations in sensor fabrication and application. In this work, a salicylic acid- containing “SiO2/propylene carbonate” liquid photonic crystal (Sal-LPC), as a new photonic sensing material, was developed to demonstrate reliable measurement of Cu2+ in water. When the Sal-LPC reagent was mixed with the test sample, the Cu2+ promoted the release of H+ from Sal and shrank the photonic crystal lattice, so that the Cu2+ concentration could be determined by the reflection blueshift of liquid PC. The Sal-LPC reagent showed a stronger response to Cu2+ than to other cations, and its sensitivity and measurement range could be optimized by the particle fraction and Sal dosage. Compared to traditional PC hydrogel sensors, the liquid PC reagent was composed of colloidal particles Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Received 2nd February 2020 and responsive species, which required no strict control in synthesis. More importantly, the optical Accepted 26th February 2020 response of the liquid PC reagent was scarcely affected by changes in synthesis, storage, or application, DOI: 10.1039/d0ra01014f and it could interact with the analyte quickly and quantitatively, which ensured accurate and repeatable rsc.li/rsc-advances measurement in either chemical analysis or environmental monitoring. Introduction porosity,13 magnetic elds,14–16 humidity,17–20 pH value,21,22 ions,23–26 vapor,27–29 microbes,30 and biomolecules.31–34 Such Photonic crystals (PCs) are periodic dielectric structures which enthusiasm should continue in the coming years, considering This article is licensed under a forbid the propagation of electromagnetic waves of certain the practical applications of these materials. frequency ranges. Once an external light source is projected Among all PC sensors, a chemical sensor for the detection of onto the PC, it shows characteristic reection, transmission, ions and molecules has gained a lot of attention, not only – Open Access Article. Published on 17 March 2020. Downloaded 9/23/2021 3:41:31 PM. and structural colors due to the modulation of light by the because it was developed very early35 37 and studied extensively photonic bandgap. A responsive photonic crystal (RPC) is one in history, but also because it was considered to be a prototype kind of smart PC material, which changes periodic dielectric for other PC sensors. Generally, a PC-based ion sensor was structures and thereby optical properties along with a change in composed of colloidal crystals embedded in hydrogels or external stimuli.1–5 Due to the intrinsic relationship between inverse opaline hydrogels, whose polymer networks were cova- external stimuli, PC structure, and optical signals, which form lently anchored with the recognition groups. Once the target a basic framework for a sensing mechanism, these responsive ions entered the PC gels, they would react with the recognition photonic crystals have been broadly investigated as PC sensors. group, shrink/expand the gel, and eventually lead to a change in Many chemical or physical parameters in which people are reection signals. For example, Asher et al. developed an 8- interested have been measured, either qualitatively through hydroxyquinoline modied PC hydrogel for the detection of color changes or quantitatively by shisinreection peaks. In Cu2+ in drinking water.38 As the Cu2+ concentration increased, the past decade, researchers have developed various PC sensors the binary coordination complex between Cu2+ and 8-hydrox- to detect temperature,6–8 mechanical force,9–11 viscosity,12 yquinoline changed to a univalent complex and the hydrogel swelled so that the Cu2+ concentration could be measured by the redshi of the reection peak. In another instance, a crown School of Chemistry and Molecular Engineering, Shanghai Key Laboratory of Green ether graed PC hydrogel was reported to detect the smallest Chemistry and Chemical Processes, East China Normal University, Shanghai metal ion Be2+ based on the selective chelation of crown ether 200062, China. E-mail: [email protected]; Tel: +86-21-62224356 and cations.39 The Be2+ concentration could be measured † Electronic supplementary information (ESI) available: OM and SEM images of liquid PCs, inuence of Cu2+ or Sal upon reection signals, reection blueshi according to the re ection redshi of the PC hydrogel, as the of liquid PCs with different particle volume fraction, reproducibility in chelation would promote the swelling of the hydrogel and measurements of reection blueshi, the composition of all liquid PCs in this thereby the expansion of the PC lattice. Although polymeric PC work, the formation constant of metal–salicylic acid complex, and the based chemical sensors had already been demonstrated disassociation of Sal promoted by metal cation. See DOI: 10.1039/d0ra01014f 10972 | RSC Adv.,2020,10,10972–10979 This journal is © The Royal Society of Chemistry 2020 View Article Online Paper RSC Advances through the detection of different ions and molecules, there (Fe(NO3)3$9H2O) were all purchased from Sigma-Aldrich Co. were still disadvantages for the traditional PC sensor. The Ltd. All chemicals were used as received without further fabrication of PC hydrogel sensors usually required multiple treatment. steps, including colloidal assembly, hydrogel polymerization, and recognition group modication. The change in optical Preparation of Sal-LPC detection reagent signals was affected by the loading amount of recognition Firstly, monodispersed SiO colloidal particles were prepared by group, the xation of PC lm on the substrate, and even the 2 a modied Stober¨ method. Aer washing with ethanol three testing position. Therefore, precise and repeatable measure- times, SiO colloidal particles (0.025 cm3) were dispersed in ment results could be a challenge for the commercialization of 2 ethanol (1 mL) to form a stable colloidal dispersion. Propylene these traditional sensors. In previous work, most studies carbonate (75 mL) and the ethanol solution of salicylic acid focused on the sensitivity and measurement range, but very (0.5 mM, 10 mL) were then added to the above solution and little attention was paid to repeatability among individual mixed with it by a vortex mixer. Subsequently, the mixture was sensors. transferred to an oven at 90 C for 2 h, where ethanol was In this work, we would like to demonstrate a new kind of evaporated to produce a salicylic acid-containing SiO /PC liquid liquid PC based chemical sensor through its detection of Cu2+ 2 photonic crystal (Sal-LPC) detection reagent with a total volume in water. A liquid PC was prepared by the thermal evaporation of 100 mL. The preparations of Sal-LPC reagents with different induced supersaturation of colloidal particles followed by their Sal concentrations, particle volume fractions, and particle self-assembly in solution.40 Compared to traditional methods, diameters were the same as in the above procedures, except for this was an efficient way to obtain highly crystalline colloidal the indicated changes made in preparation. PCs at a large scale, which guarantees their application in related devices. Liquid PCs have a vast potential to serve as 2+ detection reagents since their optical signals are sensitive to Working curves for measurement of Cu concentration 2+ Creative Commons Attribution-NonCommercial 3.0 Unported Licence. many physicochemical parameters in solution, including The working curve for the measurement of Cu concentration particle volume fraction, pH value, ionic strength, solvent (CCu2+) was established by measuring the reection wavelength polarity, and viscosity. Here, a salicylic acid-containing “SiO2/ change of the liquid PC reagent when it was mixed with an propylene carbonate” liquid photonic crystal (Sal-LPC) is an aqueous solution of Cu2+ at different concentrations. In accurately sensitive reagent to Cu2+ concentration, because the a typical test, the Sal-LPC reagent (100 mL) was mixed with Cu2+ promotes the release of H+ from Sal and shrinks the deionized water (10 mL) to form a homogenous suspension. The photonic crystal lattice so that the Cu2+ concentration can be Sal-LPC reagent with deionized water was kept for 5 min to determined by the reection blueshi of the liquid PC. The Sal- ensure complete interaction. Some of the above mixed solution LPC reagent had a stronger response to Cu2+ than to other (5 mL) was taken out and sandwiched between two glass slides to This article is licensed under a cations, and its measurement range could be optimized by the form a liquid lm with a thickness of 100 mm. Aer being kept Sal dosage. Compared to traditional PC hydrogel sensors, the for 5 min, liquid colloidal PC spontaneously precipitated from liquid PC reagent was composed of a mixed solution of colloidal the liquid lm and its reection wavelength was measured and l m Open Access Article. Published on 17 March 2020. Downloaded 9/23/2021 3:41:31 PM. particles and responsive species, which required no strict recorded as 0. On the other hand, the Sal-LPC reagent (100 L) control over synthesis and modication of photonic crystals. was mixed with an aqueous solution of Cu2+ (10 mL) to form More importantly, the optical response of the liquid PC reagent another homogenous suspension and a new liquid PC, whose was scarcely affected by changes in synthesis, storage, or reection wavelength was measured to be l0. Then, the reec- 0 application, and it could interact with the analyte quickly and tion wavelength change (Dl ¼ l À l0) caused by the introduc- quantitatively, all of which ensured an accurate and repeatable tion of Cu2+ could be determined according to the two response for this new sensing material.