A significant blue shift of the luminescence of CdSe/ZnS quantum dots (QD) conjugated with functionalized multi-wall carbon nanotubes (CNT) is investigated. The observed Stark shift is due to the local electrostatic field induced by the carboxylic anions on the CNT surface. A theoretical model is developed to evaluate the contribution of the surface charges to the observed spectral shift ( up to 0.59 eV). Based on the present model, the Stark shift provides an effective method for evaluating the density of carboxyl groups on the surface of functionalized CNTs and can be used as a charge detector for QD-nanostructures. (C) 2008 Elsevier B. V. All rights reserved.