Nanotechnology is an interdisciplinary field of research involving allied disciplines namely physics, chemistry, biology, electronics, medical, food and agriculture. The synthesis of metal oxide nanoparticles especially ZnO has received gigantic attention due to their global medical and industrial applications [1, 2]. Zinc oxide is an n-type semiconductor which has broad band gap (~ 3.37eV) and substantial exciton binding energy ~ 60 meV at room temperature. It is especially attractive for biological applications like sensing, biological labelling, gene delivery, cell imaging and so on. The other properties of relevance like high electron mobility, high thermal conductivity and good transparency make them candidates of choice in the field of electrochemistry [3, 4].
Number of chemical and physical processes such as co-precipitation, thermal decomposition, sol-gel, wet chemical, direct precipitation, microwave assisted combustion have been used for the synthesis of ZnO nanoparticles [5–8]. Among the various methods, green synthesis is promoted as it is cost effective, bio safe, biocompatible and eco-friendly approach employing micro-organisms, extract of different parts of plant, DNA and proteins. By virtue of their eco-friendly and sustainable production, biological methods are now being used to synthesise ZnO nanoparticles with desired characteristics [9–11].
Different plant extracts namely Cnidoscolus aconitifolius [12], Ricinus communis [13], Calotropis gigantea [14], Plectranthus amboinicus [15], Vitex negundo [16] among others have been used to carry out the green synthesis of ZnO NPs. The phytochemicals such as flavones, terpenoids, polysaccharides, ketones, aldehydes, carboxylic acids, and amides found in plants are involved in the bio-reduction and stability of nanoparticles. The functional groups present increases their capacity to reduce metal ions [17].
Mussaenda frondosa is a significant tropical plant of family Rubiaceae. The plant has several medicinal benefits and is extensively used as astringent, expectorant in jaundice, wound healing, leprosy, ulcers, hyperacidity, antimicrobial, and protective asthma [18].
Dopamine (DA) is an important neurotransmitter present in mammalian central nervous system which affects brain processes that control movement, emotional response. Changes in concentration levels of DA may give rise to several diseases such as Parkinson’s disease, Schizophrenia and HIV infection senile dementia. Detection of Dopamine has become an interesting field of research [19–21]. Therefore, it is in demand to develop simple and fast methods to determine Dopamine. Since Dopamine is an electrochemically active compound, its electrochemical behaviour can be studied using voltametric methods. The voltametric techniques are mostly used due to their very good selectivity, high sensitivity, rapid response, good accuracy, their simplicity and low cost. Generally, the rate of electron transfer is slow at the bare electrode surface, therefore the fabrication of cheap modified electrode for determination of Dopamine is investigated by researchers. The modification has to be done by mixing the carbon paste with the modifier in a mortar. It is possible to use various inorganic or organic substances as a modifier. Hence, fabrication of modified carbon paste electrode with excellent catalytic applications is of great importance [22–24].
In the present work, we successfully synthesised ZnO NPs using Mussaenda frondose leaf extract. The fabrication of ZnO modified carbon paste electrode and the study of electrochemical behaviour of Dopamine was carried out using cyclic voltammetry. The ZnO modified carbon paste electrode showed excellent electrocatalytic oxidation of dopamine showing its application in the development of sensors. In these times, greater the functionality of a product greater the acceptance in an ever-demanding society. Hence, we aimed at testing the antibacterial activity of these nanoparticles against bacterial pathogens namely Klebsiella sp. and S. aureus. Klebsiella sp. is a Gram-negative bacteria implicated in healthcare-associated infections, such as pneumonia, bloodstream infections, wound or surgical site infections, and meningitis [25]. Staphylococcus aureus on the other hand is an infectious agent implicated for the worldwide causes of morbidity and mortality. It causes diseases, ranging from moderately severe skin infections to fatal pneumonia and sepsis [26]. The biocidal activity of the nanoparticles was ascertained using the agar cup diffusion assay.