Ag colloids can be prepared by using various methods: chemical, electrochemical, Îł-radiation photochemical, laser ablation, etc. The simples preparation of Ag colloids is chemical reduction of silver salts by sodium citrate or sodium borohydride. Preparation of Ag colloid by borohydride reduction method requires only addition of sodium borohydride into an aqueous solution of silver salt like AgNO3. However, great care must be taken to obtain a stable and reproducible colloid. The purity of water and reagents, cleanliness of the glassware and personal skill are critical parameters. We shall follow the trisodium citrate (C6H5O7Na3) reduction method here, which gives little better control over the stability due to the surface adsorption of relatively large citrate anions. The reduction reaction could be expressed as follows:

4Ag+ + C₆H₅O₇Na₃ + 2H₂O (l) → 4Ag⁰ + C₆H₅O₇Na₃ + 3Na⁺ + H⁺ + O2 ↑

n Ag⁰ → (Ag⁰)n

where n is the number of atoms aggregate together to form a colloid particle.

The colloid is made of nanoscale Ag particles suspended in the aqueous solution. The particles acquire negative charge and remain suspended in solution due to the repulsion caused by the surface adsorption of the citrate anions. Concentrations of AgNO3 and reducing agent, reaction time, solution temperature, etc. influence the particle size.

The physical and chemical properties of colloidal particles change dramatically when their size is reduced to tens of nanometers. This effect is due to the increasing influence of the surface of the particles, since adsorption, reactivity, etc. are highly surface-sensitive properties. A particle interacts with its environment through its surface. The larger the particle surface area, the higher the contacts with its environment. The large surface of a fine particle often becomes structurally and compositionally different from the bulk counterpart due to surface relaxation and reconstruction. Due to the finely divided states, such particles provide different crystallographic facets, ample defects, kinks, steps, corners and vertices. These effects lead to a radical alteration in chemical reactivity. For example, when a saturated solution of mercuric chloride (HgCl2) is added to a yellow coloured solution of Ag colloid, the yellow colour disappears and a milky white colour appears. This occurs due to the reaction between colloidal Ag and HgCl2 producing precipitates of silver chloride (AgCl) and mercurous chloride (HgCl).

(Ag⁰)_n (colloid) + HgCl₂ → AgCl (↓) + HgCl (↓)

To demonstrate this property, first we shall prepare Ag colloid and then study their chemical behaviour in the presence of HgCl2. Ag colloids find many applications such as anti-microbial agents, biomolecular labels, surface-enhanced Raman scattering (SERS) substrates, and so on.