Sains Malaysiana 51(4)(2022): 1197-1212

http://doi.org/10.17576/jsm-2022-5104-20

Microwave Synthesis of Blue Emissive N-Doped Carbon Quantum Dots as a Fluorescent Probe for Free Chlorine Detection

(Sintesis Gelombang Mikro N-Terdop Titik Kuantum Karbon Pancaran Biru sebagai Prob Pendarfluor untuk Pengesanan Klorin Bebas)

MELISSA ASHA LARSSON¹, PRAVENA RAMACHANDRAN¹, PURIM JARUJAMRUS^2,3 & HOOI LING LEE^1*

¹Nanomaterials Research Group, School of Chemical Sciences, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia

²Department of Chemistry and Centre of Excellence for Innovation in Chemistry, Faculty of Science, Ubon Ratchathani University, Ubon Ratchathani, Thailand

³Nanomaterials Science, Sensors & Catalysis for Problem-Based Projects, Faculty of Science, Ubon Ratchathani University, Ubon Ratchathani, Thailand

Received: 8 June 2021/Accepted: 8 September 2021

Abstract

Blue emissive N-doped carbon quantum dots (N-CQDs) were prepared through a convenient and sustainable microwave synthesis method using citric acid monohydrate (CA) and urea as carbon and nitrogen sources, respectively, with an optimum molar ratio of 1:3 (CA:Urea). The surface functional groups, morphology, and hydrodynamic characteristics of N-CQDs were analysed with Fourier-transform infrared (FTIR) spectroscopy, high-resolution transmission electron microscopy (HRTEM) and dynamic light scattering (DLS), respectively. The as-synthesised N-CQDs with a quantum yield of 14.8%, exhibited excitation-independent fluorescence emission at 443 nm due to surface-state-induced fluorescence, with an optimum excitation wavelength at 360 nm. The N-CQDs were spherical, with an average particle size of 7.29 ± 3.91 nm based on HRTEM analysis. However, DLS analysis showed that the hydrodynamic size (293.0 ± 110.8 nm) was larger than the average particle size due to the presence of hydrophilic polymer chains and abundant surface groups on the N-CQDs. The free chorine-induced fluorescence quenching of N-CQDs at pH 9 denotes the sensitivity of N-CQDs towards detection of free chlorine in the form of hypochlorite (ClO^-) ion, providing the limit of detection (LOD) of 0.4 mM and limit of quantification (LOQ) of 1.2 mM. The fluorescence quenching effect in the N-CQDs caused by the quencher (ClO^-) is attributed to the dynamic quenching mechanism, via an intersystem crossing. The low selectivity of N-CQDs towards various ions justified N-CQDs' selectivity as a free chlorine fluorescent probe that can be used for wastewater testing due to its high range sensitivity.

Keywords: Fluorescent probe; free chlorine detection; microwave synthesis; N-doped carbon quantum dots (N-CQDs)

Abstrak

Pancaran biru titik kuantum karbon terdop N (N-CQDs) telah disediakan melalui kaedah gelombang mikro yang mudah dan lestari dengan menggunakan asid sitrik monohidrat (CA) dan urea masing-masing sebagai sumber karbon dan nitrogen dengan nisbah molar optimum 1:3 (CA:Urea). Kumpulan berfungsi permukaan, morfologi dan ciri hidrodinamik N-CQDs telah dianalisis dengan spektroskopi transformasi Fourier inframerah (FTIR), mikroskop pancaran elektron resolusi tinggi (HRTEM) dan penyerakan cahaya dinamik (DLS), masing-masing. N-CQDs yang disintesis dengan 14.8% hasil kuantum telah mengeluarkan pelepasan pendarfluor pengujaan-bebas pada 443 nm disebabkan pendarfluor teraruh keadaan permukaan dengan gelombang pengujaan optimum pada 360 nm. N-CQDs adalah berbentuk sfera dengan purata saiz zarah 7.29 ± 3.91 nm berdasarkan analisis HRTEM. Namun, analisis DLS telah menunjukkan bahawa saiz hidrodinamik (293.0 ± 110.8 nm) adalah lebih besar daripada purata saiz zarah disebabkan kehadiran rantaian polimer hidrofil dan banyak kumpulan berfungsi permukaan pada N-CQDs. Lindapan pendarfluor N-CQDs teraruh klorin bebas pada pH 9 menunjukkan kepekaan N-CQDs terhadap pengesanan klorin bebas dalam bentuk ion hipoklorit (ClO^-), yang memberi had pengesanan (LOD) 0.4 mM dan had pengkuantitian (LOQ) 1.2 mM. Kesan lindapan pendarfluor di N-CQDs yang disebabkan oleh pelindap (ClO^-) adalah dikaitkan dengan mekanisme lindapan dinamik, melalui lintasan antara sistem. Kepekaan N-CQDs yang rendah terhadap beberapa ion mewajarkan keterpilihan N-CQDs sebagai prob pendarfluor klorin bebas yang boleh digunakan bagi pengujian air sisa disebabkan julat kepekaan yang tinggi.

Kata kunci: Pengesanan klorin bebas; prob pendarfluor; sintesis gelombang mikro; titik kuantum karbon terdop N (N-CQDs)

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^*Corresponding author; email: hllee@usm.my