Taking for instance, nanoparticle synthesis through bioreduction involves reduction of metal ions by the polar groups present in the compounds. But when an electron is transferred from a polar group to the metal ion, there must develop a positive charge on the polar group (a positive charge on an electronegative atom- highly unstable!). How does this get neutralised?
1. Basically: The reductant of metal ions is organic material, which is oxidised. Most organic groups including biological material are not fully oxidised. E.g. any C-H containing molecule can be oxidised up to CO2 and H2O.
2. When it comes to a mechanistic description (your question) I would say that positive charges on electronegative atoms are reactive (avoid the term "unstable", it has no scientific meaning in chemistry) and as you could expect, their reactivity increases along the series C(+) < N(+) < O(+) < F(+).
At the same time, this means that C(+) particles, which we call carbocations, are the most stable in this (very general) series. These carbocations are important species in many organic reactions (including SN1-type nucleophilic substitutions) and they are relatively stable. Some of them have been isolated.
Thus, if any organic group is oxidised and such a positive charge appears, the follow-up chemistry will compensate this. E.g. carbocations react rapidly with hydroxide (OH-) forming an alkohol R-OH function. The OH- can stem from water and a proton (H+) is then left over.
This explains quite well why the presence of water is essential for redox reactions even if the H2O molecule is not involved directly in the redox process (producing O2 or H2 out of it).
Hope this helps.
See the silver and AuCl4- ion reduction mechanism using rutin.
Article Kinetics and mechanism of the interaction between HAuCl4 and rutin
Article Investigation of interaction between silver ions and rutin i...
Here is a picture shows the suggested mechanism; still no conclusive proof, yet.
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