Carbon dots (CDs) are zero-dimensional quasi-spherical nanoparticles endowed with excellent advantages including good luminescence features, photostability, low cytotoxicity, remarkable aqueous solubility, favourable biocompatibility, low risk to environment and great flexibility in surface modification. Fluorescent CDs that can selectively respond to specific inorganic/organic target molecules in environmental and biological samples are of prime significance amongst the new generation intelligent sensors due to the critical involvement of different ions/molecular species in not only human health, but also in environment processes. In this context, preparation of CDs from bioprecursors has immense significance due to the involvement of green principles, inexpensive, clean, nontoxic, easily accessible, renewable and large-scale production can be realized. This article aims at exploring different types of green raw materials including plant biomass, animal products, food items and waste materials as carbon sources for the synthesis of both undoped and doped CDs. The emphasis is given on different synthetic approaches adopted for improving the quantum yield without any chemical modification, the characterization techniques, mechanistic origin of photoluminescence and fluorescence response mechanisms involved in the sensing action towards various analytes. The significant benefits and limitations of CDs obtained from eco-friendly precursors through green approaches are summarized. Various challenges and the future prospects of these carbonaceous nanomaterials as sensors are also discussed.
Graphical abstract
Mild steel (a low carbon steel) is an affordable engineering material used for many purposes in various environments including mild acidic environment with some precautions. The corrosion behaviour of mild steel (MS) in 0.5 M H2SO4 and 0.5 M HCl, in the temperature range (303–323 K) without and with the inhibitor N-[(3,4-dimethoxyphenyl) methyleneamino]-4-hydroxy-benzamide (DMHB), was investigated using Potentiodynamic polarization and Electrochemical impedance spectroscopy (EIS) techniques supplementing with surface characterization study using scanning electron microscope (SEM) and atomic force spectroscopy (AFM). Experimental observations were found to be in agreement with Density functional theory (DFT) calculations. The inhibition efficiency increases with increase in DMHB concentration and showed maximum inhibition efficiency of 86% in 0.5 M H2SO4 and 81% in 0.5 M HCl, respectively, at concentration of 3 × 10─3 M at 303 K. The inhibition efficiency of DMHB obtained relatively at its lower concentration (3 × 10─3 M) compared to other reported related compounds confirms its potential towards corrosion inhibition. The variation in the kinetic and thermodynamic parameters indicated physisorption of DMHB on MS and its mixed type inhibitive action followed Langmuir’s isotherm model. DFT calculations go along with the experimental results, signifying the potential corrosion inhibition behaviour of DMHB for MS in both the acid media.
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