In recent times, agricultural production has skyrocketed due to the impact of the green revolution and technological advancement consequent to accelerated population growth and increased demand for agri-food products. This global increase in agricultural production has put immense pressure mainly due to the inappropriate management of the generated agro-waste [1]. Crop residues as well as agro-industrial wastes constitute the primary waste materials from the agricultural production system. The sustainable management of natural resources could effectively be attained through prevention, recycling, and reusing [2]. Valorization of agri-waste products could be achieved mainly by bio-refinery, extraction procedures, biotechnology, and nanotechnology, which can serve as valuable precursors for biofertilizers, and biofuels [3]. Lately, nanotechnology has been identified as a potential practice for the value addition of materials emanating from the agro-industry, of which eco-friendly cost-effective methods can have a substantial value [4].

High surface area-to-volume ratio and conductivity are among the specific features of metal and metal oxide nanoparticles (NPs). Moreover, such NPs were reported to exhibit biomedical applications as agents in drug delivery, diagnostics, sensors, cosmetics, agriculture, catalysis, light emitters, optics, electronics, and the food industry [5]. Due to the use of hazardous chemicals in the manufacturing of NPs, there has been a lot of recent research focused on an environmentally safe and economically viable green synthesis method utilizing plants, plant extracts, bacteria, and fungi [6]. Remarkably, several agro-waste materials and their extracts have been explored as potential stabilizers and reducers for the green synthesis of NPs. A wide variety of metal (silver, copper, gold, zinc) and metal oxide (CuO, NiO, Fe2O3) NPs were synthesized from agro-waste products using a green synthesis approach [[7], [8], [9]]. Precisely, several NPs (Ag, CuO, CdO) have also been synthesized by hydrothermal method and were found to exhibit promising biomedical applications including antibacterial activity [10]. Biohydrothermal synthesis is a solution-based approach performed at high temperatures and vapor pressure that results in the formation of NPs with controlled dimensions [11]. Being a compound with a ‘generally recognized as safe’ (GRAS) status, zinc oxide (ZnO) has gained wide attraction in the fields of biomedical research [12]. ZnO NPs synthesized by bio-hydrothermal approach were reported to be an antibacterial candidate against several bacterial pathogens [13]; however, the activity against multi-drug-resistant (MDR) strains remains unexplored.

Coffee, one of the popular beverages worldwide, is prepared from roasted beans. Coffee husk, the major agro-waste product (30–50 %) obtained during the dehulling process [14], would adversely impact the soil and water because of high organic matter, caffeine, and phenolic compounds [15]. The bioactive molecules (caffeine, gallic acid, protocatechuic, and chlorogenic compounds) present in the coffee husk might serve as an excellent reducing agent for the nano-fabrication process [16,17]. Export of coffee and products worth more than 40 billion dollars per year [18]. India, which ranks seventh in the world, produces about 3 % of the world's total coffee exports. The production of coffee in India can mainly be attributed to small-scale farmers [19]. Moreover, the ‘Monsooned Malabar Robusta’ variety of coffee (Coffea canephora robusta) has been recognized as the geographical indicator (Indication No.: 114) of Wayanad district of Kerala state in India. The fact that almost 650 kg of residue is produced for every ton of coffee consumed illustrates the environmental impact of the large-scale production [20]. Thus, proper management of the agro-waste generated is an important aspect that needs urgent attention. To the best of our knowledge, the hydrothermal synthesis of ZnO NPs using coffee husk extract has seldom been reported. Furthermore, ZnO NPs fabricated using coffee husk extract for exploring antibacterial potential provide a naïve area of research. Hence, this study has been designed to synthesize and characterize ZnO NPs by a hydrothermal approach using the husk extract of ‘Monsooned Malabar Robusta’ variety of coffee as an approach for valorization and further evaluate the antibacterial efficacy against MDR strains of bacterial pathogens of public health importance along with their antioxidant properties.