On-site colorimetric detection of adulterated gasoline using highly reflective 1D photonic crystal sensors based on photo-crosslinked polymer–titania hybrids

Highlights

• Highly reflective 1D PC was developed with novel polymer–titania hybrid and polymer.

• Fabricated 1D PC demonstrated excellent recyclability.

• 1D PC exhibited no change in color in pure commercial gasoline.

• Reflectance colors of 1D PC changed rapidly when immersed in organic solvents.

• Colorimetric detection of adulterated gasoline was achieved using 1D PC.

Abstract

The adulteration and distribution of pure commercial gasoline remain a global problem because of the lack of a simple method for the rapid on-site detection of adulterated gasoline. Therefore, herein, we propose a simple method, based on highly reflective one-dimensional photonic crystals (1D PCs), for the on-site rapid colorimetric detection of adulterated gasoline by the naked eye. A photo-crosslinked copolymer–titania hybrid (Ti70) with a high refractive index and a photo-crosslinked methyl acrylate (MA)-based polymeric material containing 4-benzoylphenyl acrylate (BPA) (P(MA-co-BPA)) with a low refractive index were synthesized, characterized, and used in the fabrication of an 11-layered 1D PC sensor (Ti70/P(MA-co-BPA) PC). Ti70/P(MA-co-BPA) PC exhibited no color change in pure commercial gasoline. However, when Ti70/P(MA-co-BPA) PC was immersed in methanol, xylene, toluene, and benzene, it rapidly turned from bluish-purple to blue, sky-blue, greenish-yellow, and reddish-orange, respectively, and had high reflectivities. Additionally, when immersed in binary (gasoline/toluene and gasoline/methanol) and ternary (gasoline/toluene/methanol) mixtures, Ti70/P(MA-co-BPA) PC demonstrated significant color changes within short periods (~2 min). Ti70/P(MA-co-BPA) PC exhibited excellent recyclability, which was verified via repeated swelling/deswelling tests for 10 cycles. These results indicate the applicability of the proposed novel Ti70/P(MA-co-BPA) PC for the rapid on-site colorimetric detection of adulterated gasoline by the naked-eye. We believe that the findings of this study will provide considerable insight for developing simple and high-performance 1D PC sensors for practical use and will also serve as a basis for exploring their potential applications.

Introduction

Recently, petrol prices have increased to an all-time high due to global economic crises, including the Russia–Ukraine war [1]. Although petrol is expensive and is being replaced by renewable energy sources, the worldwide demand for petrol, specifically, gasoline, is still high [2]. The demand may encourage the illegal production and distribution of adulterated gasoline. Adulterated gasoline can be used in economic crimes: a small quantity of high-quality gasoline is converted into a large quantity of inferior gasoline, and therefore, illegal profits are generated by the distribution of cheap adulterated gasoline [3]. Pure commercial gasoline is mainly composed of branched alkanes (C₄–C₁₀), straight-chain alkenes (C₄–C₈), and aromatics, and contains small quantities of cycloalkanes and straight-chain and cyclic alkenes [2]. In contrast, adulterated gasoline is a mixture of pure commercial gasoline and cheap organic solvents, for example, toluene and methanol [4], [5], [6]. The use of adulterated gasoline causes environmental pollution, poses carcinogenic health risks to living organisms, and leads to the malfunction of gasoline-powered internal combustion engine-based machines [7], [8]. Nevertheless, the distribution of adulterated gasoline remains a global problem because of the lack of a simple method for its rapid on-site detection. Conventional analytical instruments including high-performance liquid chromatography, gas chromatography, and Fourier transform infrared (FTIR) spectrometers can be used for the highly precise and reliable detection of adulterated gasoline [9]. However, these instruments cannot be utilized for the rapid on-site detection of adulterated gasoline because they are energy-consuming and occupy large spaces [10], [11], [12]. Numerous viable alternative instruments, such as semiconductor-based portable sensors, [8], [9], [13] fluorescent dye-based paper strips, [14] polydiacetylene-based microfibers, [6] nanoparticle-based optical sensors, [15] cataluminescence sensors, [16] and one-dimensional photonic crystals (1D PCs), [17], [18], [19] have been developed for the on-site detection of adulterated gasoline, and are continuously refined and improved. Nevertheless, the development of sensors that are simpler, less expensive, and are more reliable than conventional sensors is still required for the rapid on-site detection of adulterated gasoline in real-world applications.

The 1D PC sensor is one of the most promising sensors for rapid on-site detection. 1D PCs, also called Bragg stacks, are periodic nanostructures that are constructed by alternately stacking layers of two different materials with different refractive indices in one direction: a high refractive index layer (HRIL) and a low refractive index layer (LRIL). Over the past decade, several advantages of 1D PCs have been reported: simple structures, easy and low-cost fabrications, rapid response rates, sensitivities to specific environmental changes, and tunable optical properties [20]. Hence, 1D PCs have been established as reflective optical sensors for the detection of various analytes by the naked eye and do not require an external power source. Owing to the abovementioned advantages, 1D PC-based sensors are promising candidates for the on-site detection of adulterated gasoline in real-world applications [18], [19]. The 1D PC structures function by allowing or forbidding the propagation of incident light, depending on its wavelength.

The forbidden wavelength range is called the photonic stop-band (PSB), and the light corresponding to the PSB is reflected. Furthermore, the 1D PCs can exhibit multiple colors by the facile control of the PSB, and exclude any dyes and pigments that require complicated chemical syntheses and additional strategies, to enhance the visibility for naked-eye detection [21], [22], [23]. The peak wavelength range and reflectivity of the PSB in 1D PCs can be estimated via (1), (2), [24] respectively. Therefore, the refractive index and thickness of each HRIL and LRIL, constituting the 1D PCs, determine the wavelength of the reflected colors. Additionally, the reflectivities of 1D PCs can be modified by regulating the difference between the refractive indices (Δn) of the HRIL and LRIL, and the number of stacking layers.

$${\text{λ}}_{max}\text{ = }2\text{(}{n}_l{d}_l+n_h{d}_h),$$

$$R={\left(\frac{1-Y}{1+Y}\right)}^2\times 100\%,\mathrm{where}Y={\left(\frac{n_h}{n_l}\right)}^{N-1}\frac{{n_h}^2}{n_s},$$

where n_h, n_l = refractive index of the material (n_h > n_l);.

n_s = refractive index of the substrate;.

d_h, d_l = thickness of the material; and.

N = number of periods comprising the distributed Bragg reflectors.

Recently, perfluorinated polymers with extremely low refractive indices (1.3–1.4) have been reported as potential candidates for the LRILs of 1D PCs [25], [26]. Moreover, in recent studies, inorganic HRILs, such as titania (TiO₂) and ZnO, have been successfully applied in 1D PC sensors. The use of these materials in 1D PC sensors has considerably increased the reflectivities of the 1D PC sensors by approximately 80 % due to the large Δn. Therefore, 1D PC sensors comprising TiO₂ and ZnO can be synthesized cost-effectively, using only a small number of total stacking layers (6−10) [18], [27], [28]. In particular, polymer–inorganic hybrid materials possess optimal characteristics, for application as HRILs, compared with those of their individual counterparts. The inorganic components enhance the refractive indices of the hybrid materials, and the polymer components improve the stabilities of the materials by forming crosslinked networks; the compatibilities of the hybrid materials with other materials are enhanced by controlling their hydrophilic/hydrophobic character. Additionally, dispersion techniques are rarely required to fabricate optically transparent homogeneous thin films using the polymer–inorganic hybrid materials, compared with instances when inorganic materials are solely used; thus, undesirable light scattering can be easily avoided in the thin films [29], [30]. However, although these hybrid materials appear desirable for the development of highly reflective hybrid 1D PCs such as inorganic–hybrid, polymer–hybrid, and hybrid–hybrid PCs, it remains challenging to secure compatibility between those heterogeneous materials. In addition, a tailored design strategy of hybrid materials is required to develop hybrid material-based 1D PC sensors capable of detecting specific analytes, but related studies are still lacking.

In this study, we designed and fabricated highly reflective 1D PC sensors for the naked-eye colorimetric detection of methanol- and toluene-adulterated gasoline samples. To the best of our knowledge, this study is unprecedented because novel polymer–inorganic hybrid materials have not previously been used as the HRILs in 1D PC sensors, to obtain high reflectivity and excellent recyclability. Moreover, a methyl acrylate (MA)-based polymeric LRIL, was utilized as a counterpart to HRIL, to attain the desired selectivity and sensitivity for the target analytes. Both the HRIL and the LRIL contained photocrosslinking groups that prevented the degradation of these layers during the repeated swelling/deswelling tests [29]. Thereafter, the layers were used in the preparation of a novel 11-layered 1D PC sensor. To verify whether the 1D PC sensor was suitable for the on-site detection of adulterated gasoline, it was immersed into different solutions, namely, gasoline, methanol, toluene, xylene, benzene, and their mixtures. Furthermore, the optical properties, response times, and recyclability of the 1D PC sensor were analyzed.