Some chemicals, such as sodium citrate, function as both capping and reducing agents. http://www.ncbi.nlm.nih.gov/pubmed/17948996

The reducing agent provides electrons to the precursor ions. For example, in noble metals it transforms ions in the elemental metal. The capping agent stabilize the nanoparticle. Some molecules can perform both roles. You can use both compounds simultaneosly. By changing their concentrations you could control, for example, the size and shapes.

Use of reducing agent is essential when you want to make any nano-particle at its elemental state. For eg., the case of metal nanoparticle such as Au, Ag etc. You may know that when matter is sized down, there is a huge raise in its surface free energy. This leads to a huge increase in both chemical and physical interaction of the nanoparticle with its surrounding. In one hand it increases the surface chemical reactivity. The reducing agent reduces the chance of it reacting with surrounding especially to get oxidized. On the other hand, the raise in surface free energy also induces its physical reactivity. For example, inducing particle aggregation leading to an increase in the particle size.Here a size stabilizing agent also known as capping agent will take care of this. So, a reducing agent has a different role compared to that of a capping agent. Both can be used at the same time. However their individual composition differ from case to case depending on need. People also identified some agents that have the ability to do the dual role efficiently eg., certain sodium citrate and few amines. However, this is also limited to the requirement / application of the nanoparticle. Because the surface modification would influence its performance in a given application. Hope this gives you some idea now.

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A special case is found when nanoparticles are synthesized directly on a solid polymeric film as cellulose, for instance. There, no "capping" agent is needed since nanoparticles are nucleated and grown on specific sites of the solid surface (which works, simultaneously, as the chelating agent to nucleate the NP formation and the reducing agent) and not in a solution/suspension. Please read http://dx.doi.org/10.1016/j.carbpol.2011.06.067

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The main role reducing agent is giving electrons to ions to form atoms. Then these atoms will develop a particle by combing together. Capping agent /surfactant /stabilizing agent is responsible for....

prevent uncontrollable growth of particles

prevent particle aggregation

controls growth rate

 controls particle size

 allows particle solubility in various solvents

These possibilities can be achieved by

Electrostatic stabilization: Adsorption of ions to the surface creates an electrical double layer which results in a Columbic repulsion force between individual particles.

Steric Stabilization: Surrounding the metal centre by layers of material that are sterically bulky. Examples: polymers, surfactants, etc

Now you can understand reducing agent, capping agent are different and both are important in synthesis of nanoparticles. You can use both at a time, see the following examples...

Ex: Sodium borohydride (NaBH4) has a dual role that act as a reducing agent as well as stabilizing agent in silver nanoparticles formation. First it reduces metal ions to zero valent matal nanoparticles and stabilizes by being adsorbed. However, a higher concentration may result in aggregation of Ag nanoparticles due to an increased ionic strength.

Ex: Polyvinyl pyrrolidine (PVP) is a polymer that acts as a surfactant or stabilizing agent in silver nanoparticles synthesis. It prevents the particles agglomeration even in the presence of electrolytes or ionic salts.

A careful preparation of nanoparticles synthesis is depending on so many factors. Keep in this mind, and do any nanoparticles preparation depending on its conditions. So many synthesis methods are available in literature, follow it. All the best....

Thank you

Reducing agent is the chemical compound which performs reduction reaction. Typically reducing agents are used in the synthesis of metal nanostructures. They are generally reduce metal salts into pure metals, while surfactants are polymers used in the wet chemical synthesis to stabilize nanoparticles against aggregation by steric repulsion. The amount of reducing agent required in a typical reaction is normally decided by the quantity of metal salt at the starting point. While the surfactant amount depends on several parameters like, type and chain length of the polymer, the final target size of the nanoparticle, the dielectric medium in which nanoparticles are dispersed and so on. There is no thumb rule to decide what is the optimum surfactant concentration one should use. However, you can identify the one for your specific experiment. 

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In order to synthesis metal nanoparticles from salt precursor, you need to reduce the metal ions to zero valent-species. Reducing agents are used to do so. By attachment of these reduced atoms together nanoparticles are formed. But naked nanoparticles are not colloidal and chemical stable and you need some materials to make a coating or capping for your nanoparticles. Nanoparticles can be coated in the second reaction after synthesis. But, there is some interest for reduction of the nanoparticles preparation steps. In this order, you can do one-pot reaction. It means having capping agent in the synthesis reaction on the behalf of metal ions and reducing agent. Also, you can use reducing agents which can play a bifunctional role, reducing and capping.

When should one use capping agent? Is it before reducing agent or after reducing agent?

Sir, please tell some examples for the capping agents and reducing agents. 

Sodium borohydate ,Aminoacids,CTAB,BDAC polymers Ascorbic acid are reducing agents. Reduces the compound to ionic particles.Trisodium Citrate is Reducing and capping agent to prevent the binding.

Poorani mam, thanks for sharing these informations.

can all metals undergo reduction 

if yes, do they all results in the formation of nanoparticles

Can we use Trisodium citrate as a reducing agent and another chemical compound for capping? And at what time we will add capping agent? 

This link will really help you, 

http://instanano.com/characterization/theoretical/reducing-agent-calculator-nanoparticles-synthesis/

For the case where silver nanoparticles are generated from AgNO3 and the chitosan is used as reducing and capping agent, please see a recent publication:

Ana Patrícia Carapeto, Ana Maria Ferraria, Ana Maria Botelho do Rego (2017). Unraveling the reaction mechanism of silver ions reduction by chitosan from so far neglected spectroscopic features. Carbohydrate Polymers, 174, 601-609.

https://doi.org/10.1016/j.carbpol.2017.06.100

Reducing agent means reduction reagent.

While the surfactant sum relies upon a few parameters like, type and chain length of the polymer, the last target size of the nanoparticle, the dielectric medium in which nanoparticles are scattered, etc.

What is the character of pH in protein based small nano particles synthesis ?

At different pH what will happen to the nano particle ?