Quantum-Dot Complex
4.2 Introduction
The importance of long-chain unsaturated fatty acids (LCUFAs) in biological processes such as the cell membrane activity, metabolic regulation, responses in the hypothalamus, gene expressions and the functioning of anaerobic digesters is well- established.1–14 Furthermore, LCUFAs also play a vital role in the production quality of pharmaceuticals, cosmetics and dietary food products, particularly vegetable oils.1–14 An abnormal content of LCUFAs may lead to diseases in humans such as fatal diseases with symptoms akin to multiple sclerosis, coronary heart disease, chronic pain, depression and weight issues.1–14 The length of LCUFAs plays a vital role in the emulsification of several pharmaceutical and cosmetic products. On the other hand, the presence of LCUFAs, which cannot be synthesized by the human body, in dietary food products (mainly vegetable oils), compared with their saturated forms that have the same carbon chain length, is essential for lowering the serum cholesterol level (by promoting antioxidant defenses and lessening inflammatory mediators and oxidative stress) and governing cell membrane fluidity.1–14 Notably, vegetable oils (such as sunflower, edible and soybean oils) that are composed of LCUFAs, such as oleic, linoleic and erucic acids, are important day-to-day food products due to their capability of providing energy and nutritional supplements to the human body.12–14 However, the quality of vegetable oils as available commercially has a great impact on human health.12–14 A way of ascertaining this is to check for the contents of LCUFAs, such as oleic, linoleic and erucic acids, as compared with their saturated counterparts in vegetable oils.
The chemical recognition of LCUFAs from their corresponding saturated forms is challenging, owing to their flexible and long hydrocarbon chains with the limited presence of CQC group(s), surface complementarity and water insolubility.1–5 In this aspect, natural biological recognition moieties such as fatty acid-binding proteins (FABP) and cavities (e.g., FABP4 and FABP5) are known for their use in selectively recognizing LCUFAs (e.g., oleic acid, linoleic acid, etc.).1–5 Synthetic molecular receptors, such as polyaromatic receptors, polymerized liposomes, organic molecular tubes, supramolecular nanocapsules and cavitands, have displayed their usability towards the recognition of LCUFAs.1–5 For example, a polyaromatic receptor recognized oleic acid from a mixture of oleic and stearic acids while long-chain unsaturated o-fatty acids could be recognized by different forms of cavitands.1,2 However, the mentioned receptors
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need complicated fabrication strategies, timeconsuming procedures and costly instruments to recognize LCUFAs, which limit their application potential. Thus, it is important to search for new and specific synthetic platforms to discriminate LCUFAs from their corresponding saturated forms through chemical interactions.
Techniques such as gas chromatography, colorimetric assays, nuclear magnetic resonance (NMR) spectroscopy, and liquid/gas chromatography coupled with mass spectrometry (LCMS/GCMS) have been used for the detection of LCUFAs.6–11 However, the recent development of chemical recognition-based fluorometric methods provides easier, faster and cost-effective options for real sample analysis.6–11 There are several single-emitting luminescent sensors (for example, calixnaphthalene-based molecular tubes) that sense LCUFAs through luminescence quenching.8 However, their application is limited due to the drawbacks associated with singlewavelength-based optical sensors such as the variation in intensity based on the local concentration of analytes, the light source and the detector.15–18 On the other hand, the fast, precise and consistent optical detection ability of ratiometric luminescent sensors, with the supplementary benefit of a visual color change, make ratiometric luminescent sensors unique for the detection of LCUFAs.15–18 However, such ratiometric luminescent sensors – with the option of chemical interaction-based sensing – are still being developed. For example, (i) a luminescent fatty acid-binding protein has been used to ratiometrically sense oleic acid in the concentration range of 0.02–4.7 μM,9 (ii) a duplex pyrene–
cyclodextrin-based fluorescent sensor has shown its utility for the detection of oleic acid in the concentration range of 0–7.0 equivalents10 and (iii) a quantum dot–protein composite was able to sense oleic acid in the range of 10–1000 nM.11 Thus, the exploration of new ratiometric visual sensors, with two emission maxima, is of great value towards the detection of LCUFAs, mainly in dietary vegetable oils.
Incidentally, the surface engineering of bright and photostable quantum dots (Qdots), with various chemical and biological agents, has provided options for fabricating nanomaterials for white-light emission-based optical sensing and imaging.15–
20 For example, modifying the surface of Qdots with an inorganic complex has led to the formation of a nanocomposite – named a quantum dot complex (QDC) – that emits bright and photostable white light.15–20 The white-light emitting QDCs (WLE-QDCs) have shown their application potential towards the ratiometric visual detection of neurotransmitter agents like dopamine, in vitro pH, heavy metal ions (for example, Hg2+
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and Cu2+ ions), and phosphate ions in commercial fertilizer.15–18 However, the utilization of WLEQDCs towards the recognition of LCUFAs from a mixture with their corresponding saturated forms – in addition to their ratiometric visual detection – has not been explored yet. Thus, the idea of using a WLE-QDC for the combined purposes of a molecular recognition nano-platform and a turn-on ratiometric visual sensor for LCUFAs will bring newer avenues in molecular recognition and optical sensing purposes.
Herein we report the twin applications of a bright and photostable WLE-QDC, composed of Mn2+-doped ZnS Qdots and a ZnQ2 inorganic complex, towards the selective recognition of LCUFAs from their corresponding saturated forms and the turn- on ratiometric visual detection of LCUFAs in commercial vegetable oils. This was pursued by observing the variations in the luminescence characteristics (such as color, hue, emission intensity ratio (I480/I590)) of WLE-QDC upon interaction with LCUFAs.
Importantly, WLE-QDC exhibited the turn-on and ratiometric visual sensing of LCUFAs (such as Na-salt of oleic acid) with a detection limit of 0.127 μM in the linear range of 4.2–16.6 μM. Notably, WLE-QDC selectively recognized LCUFAs (such as the Na-salt of oleic acid) from a mixture with their corresponding saturated forms (i.e., the Na-salt of stearic acid). The concurrent hydrophobic and π–π interactions of LCUFAs with the quinoline moieties of the surface ZnQ2 complex (present in WLE-QDC) might play a significant role towards the observed specific luminescence changes of the WLE-QDC upon interaction with LCUFAs. Interestingly, the high selectivity of the WLE-QDC towards LCUFAs in the presence of interfering FAs (such as stearate, palmitate, laurate and geranate), anions (such as citrate, tartrate, oxalate, and fluoride) and metal ions (such as Ca2+, Mg2+, Na+ and K+) was noticed. The molecular structures of the mentioned FAs are given in Fig. A.4.1 (Appendix). Finally, the practical utilization of the WLE-QDC was investigated via the quantification of LCUFAs in commercial vegetable oils (such as sunflower, edible and soybean oils). The environmentally friendly synthesis, cost- effectiveness, low toxicity and possible specific chemical interaction of WLE-QDCs make them a preferable choice for the recognition and ratiometric sensing of LCUFAs.16–
20 The obtained results represent the first report showing the advantages of WLE-QDCs towards the selective recognition of LCUFAs from their corresponding saturated forms and the turn-on ratiometric visual detection of LCUFAs in vegetable oils through monitoring the changes in their luminescence properties.
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