It was observed that Zn

2+

and Mn

2+

additives played the activators and quenchers for the orange emission of colloidal ZnS:Mn

2+

nanoparticles

respectively. The time decay curves of Mn

2+

580 nm emission are nonexponential and become slower as Zn

2+

is added

faster as Mn

2+

added. On the basis of Langmuir isotherm model

the adsorption-desorption equilibrium constant K is obtained to be 2.9×10

4

(mol/L)

-1

. The activation process resulted from the formation of monolayer ZnS outside of nanoparticles

which blocked the nonradiative pathways related to the dangling bonds of lone pairs on surface S

2-

. It is worth noting that the blue emission is also activated on introduction of Zn

2+

indicating the original quenchers eliminated by the passivation of the surface through forming the ZnS shell can quench both the blue and the orange emission bands. It is not appropriate to describe the quenching process aroused by the addition of Mn

2+

using the standard Stern-Volmer model. Considering Poisson statistics and assuming that one Mn

2+

is sufficient for 100% quenching for orange emission

the reasonable linear plot is obtained. The average size of nanoparticles is 2.7 nm less than that calculated within the framework of effective mass approximation (EMA) model. The plausible reasons are as follows:1. the increase of E

g

owing to quantum confinement effect is overestimated by EMA;2. the assumption that one Mn

2+

is sufficient for 100% quenching of orange emission does not consist with the actual situation on the ZnS:Mn

2+

colloids. The quenching centers induced by the addition of Mn

2+

may probably be Mn

2+

themselves adsorbed on the surface of nanoparticles

which interact with the interior Mn

2+

to perform Mn-Mn energy migration within short distance. Such opinion and the formation of the manolayer of ZnS on the surface is supported by the fact that the orange emission intensity is almost unchanged with the addition of Mn

2+

after the activation process has taken place due to the addition of Zn

2+

. Surface or surface states play a crucial role on the optical properties of nanoparticles due to the relatively larger surface-to-volume ratio compared to the bulk. We expect the results may help to approach the nature of the mechanisms that surface or surface states dramatically affect the optical properties of nanoparticles.