The usage of silver nanoparticles (AgNPs) for various consumer goods like antibiotics, sensing devices, cosmetics, food, and engineered devices is increasing, due to their unique physical and biological properties1–3. Synthesis of stable AgNPs with proper distribution and properties as well as biocompatibility is important and efficient for their activities. Various methods have been reported for producing AgNPs, including chemical reduction, evaporation, laser ablation, condensation, the irradiance of laser, microwave or electrons, ionization, and photochemical routes and green synthesis using either microorganisms or plant parts4–7. Except for those green synthesis routes, most existing methods use toxic chemicals as reducing and capping agents and pose environmental problems. Some are time-consuming and require controlled conditions, complicated procedures, experienced technicians, proper hygiene, and heavy investments. Moreover, currently available AgNPs are in powder mode, hence studies involving aqueous re-dispersion of AgNPs are unable to proceed further because re-dispersion affects the nature of the prepared AgNPs. Fourier transform infrared (FTIR) spectroscopic studies ensures that the primary phytochemicals present in plants like alkaloids, polyphenols, terpenoids, glycosides etc., involves in the reduction of Ag ions (Ag+) to AgNPs due to metal ion hyper-accumulating and reductive capacity8. Thus, it is ideally desirable to adopt phytosynthesis (metal reduction using plant parts) for the production of bio benign AgNPs for a better sustainable futuristic world.
Coffee arabica plant is an excellent source of antioxidant activity because it contains chlorogenic acids (CGA), caffeic acids (CA), caffeine, trigonelline, mangiferin, carbohydrates, amino acids and rutins8–11. 5-caffeoylquinic acid (5-CQA) is one of the major CGA. As an ethno-pharmacological agent, the leaves of Coffee arabica have been using since ancient days for curing and relieving fever, cough, diarrhea, migraine, abortion-related bleeding, intestinal problems, and flu-related lungs allergies8. Green and roasted coffee beans and used coffee waste are already successfully using for preparation of nanomaterials ensuring the better bio reducing capacity as well as possibility of exploration of coffee plant towards materialist world9–10. Compared to beans the leaves of coffee are also treasure of phytochemicals8. But while trimming the branches of coffee plants, the leaves are usually discarded in the field and considered low-or no-value products. It is being neglected due to the high preference placed on seasonal and costly coffee beans. Recently, one research group has studied the structural properties and yield of ZnO NPs using the coffee leaf extract as a reducing agent12. To our best knowledge, there is no study using coffee arabica leaf extracts (CAE) to synthesize AgNPs. So, it would be interesting to synthesize CAE-reduced AgNPs (CAE-AgNPs) to produce value-added products and apply them to various fields.
Being simple, safe and cost-effective the phyto-reduced AgNPs are used as a better candidate in various molecular diagnostic processes like bio sensing13, bioimaging14, etc. The sensing ability of AgNPs is enhanced via surface plasmon resonance (SPR), spectrophotometric emission and absorption. Surface functionalization of AgNPs can be done by the introduction of various amino acids on nano surfaces, which ensures better stability as well as efficiency for the application in the fields of biomedical as sensors and catalysts. Since AgNPs show a high affinity towards nitrogen and sulfur-containing molecules15, their biological and physicochemical activities can be enhanced by the functionalization of amino acids which contain the thiol and amine groups. This is one of the reasons for the increasing demand for developing AgNPs-based biosensors for Cys which plays a crucial role to track biological pathways. Cys, the building block of proteins belonging to non-essential amino acids, is polar and uncharged and essential for cellular systems in living things. It controls detoxification, helps in protein metabolism, and suppresses the ageing of the skin by preserving its texture and elasticity by producing collagen. The imbalance in the levels of Cys in biofluids induces health disorders like cystinuria, hair loss, and growth retardation, fat loss, liver damage, white blood cell loss, etc.16–18 Hence, it is very important to know the interactive mechanisms between Cys and AgNPs for better understanding of reactivity and pre-diagnosing disorders.
Even though various techniques are adopted for the sensing of Cys like spectroscopy and voltammetry,19–25 the requirement of specific chromophores for specific analytes, various analytes pre-treatment steps, highly expensive instruments, high energy consumption and large number of synthetic solvents limits the applicability. On the other hand, some methods of Cys detection suffer disadvantages due to long reaction time, limits in detection, involvement of highly sophisticated machinery, and technicians as well as suitable physical conditions and high investment. So, colorimetric detection of an analyte using green reduced AgNPs which is simple, environmentally friendly, rapid, and cost-effective is desirable to practice.
Here in, we report a new vision of production of AgNPs using natural source CAE for the first time and explored the practical value of CAE-AgNPs, in the field of amino acid sensing; specifically, Cys.