Storage

Store silver nanoparticles at 2-8°C and protected from light. Do NOT freeze. If frozen, the silver nanoparticles will irreversibly aggregate. Aggregation of silver nanoparticles is indicated by change in color of the solution and an increase in the absorption of light in the red part of the visible spectrum, as detailed in our Silver Nanoparticle Properties Tech Note.

When stored as specified, the colloidal silver is stable for at least one year.

Handling

Large silver nanoparticles, i.e., above 60 nm in size, might sediment over time but will not affect the performance of the silver nanoparticles. Prior to use, re-suspend the sedimented particles by swirling the container until a homogenous solution is obtained.

For optimal performance and stability of the product, silver nanoparticles should not be stored diluted from the original concentration or in a bottle or flask other than that the product is supplied in. Care should be taken to use clean storage containers if other than supplied with the product.

Washing of Silver Nanoparticles

Although it is not always necessary, washing of silver nanoparticles might be required for certain applications. Silver ions released from the silver nanoparticle surface may affect some applications. The easiest way to remove such possible contaminants from the silver nanoparticle solution is by centrifugation. Centrifugal force is dependent on the size of silver nanoparticles, as listed Table I.

Notes: To prevent the silver nanoparticles from aggregating, a non-ionic surfactant such as Tween 20 should always be added prior to centrifugation.

Since non-functionalized silver nanoparticles are sensitive to salt containing buffers, re-suspension should always be performed in ultra-pure water or 2 mM sodium citrate solution, to prevent irreversible aggregation.

Procedure

1. Aliquot 1 ml of silver nanoparticle stock solution into 1.5 ml centrifuge tubes.
2. Add Tween 20 to reach a final concentration of 0.025% (w/v).
3. Centrifuge the silver nanoparticles for 30 minutes using appropriate G force, as shown in Table I below.
4. Remove supernatant and re-suspend in desired volume of ultra-pure water or 2 mM sodium citrate solution.
5. Vortex to re-disperse particles.

Table I. Appropriate G force for the centrifugation of silver nanoparticles. The conditions are based on centrifuging 1 mL of silver nanoparticles in a 1.5 ml microcentrifuge tube using a standard microcentrifuge.

Size (nm)	Speed (x g)	Time (min)
10	21,000	60
20	17,000	30
30	11,000	30
40	3,000	30
50	1,800	30
60	900	30
80	500	30
100	300	30

*For 10nm silver nanoparticles, recovery was about 50% at the specified condition. For better recovery, 1) use an ultracentrifuge to achieve higher G force; 2) use a Centricon Spin Column.