Polyacrylic acid coating of highly luminescent CdS nanocrystals for biological labeling applications

Abstract

Surface coating of highly luminescent CdS nanocrystals by polyacrylic acid was demonstrated. The method proceeded in 2 steps, (i) modification of the CdS surface by alkyl molecules and (ii) polyacrylic acid coating of the surface modified CdS. Attachment of alkyl ammonium on the CdS surface induced a phase transfer reaction from an aqueous to a non-polar phase with a yield of ∼100%. Investigating alkyl molecules with various functional groups revealed that the alkyl molecules, possessing the cation moiety, such as amine or ammonium salt, can electrostatically interact with the CdS surface. The PL of the uncoated nanocrystals was almost entirely quenched in the pH range of ∼7, while the polyacrylic acid coated nanocrystals exhibited moderate PL intensity. This PL intensity was preserved for at least several days, facilitating biological labeling application under a neutral condition.

Introduction

Semiconductor nanocrystals (NCs) have received considerable interests in biological applications owing to their superior advantages in comparison to organic dyes. These include high photostability, facile tuning of a desired fluorescence wavelength, and simultaneous excitation of multiple fluorescent NCs using a single light source [1], [2], [3], [4], [5], [6], [7], [8], [9].

NC has been applied as a biological labeling material by linking with the biological molecule, i.e. cell labeling [6], [10] or nucleotide detecting probe [2]. Since the labeling is confirmed by the fluorescence color and intensity, they are expected to be stable under the experimental environment. In the biological environment, the NCs are usually labeled and probed in neutral pH range [6], [7], [8], however the previous reports revealed that the pH influences their photoluminescence (PL) intensity. For example, CdS NCs in an aqueous solution can be photoactivated by adjusting the pH to 11 while the PL intensity decreases under an acidic condition [11]. The similar pH dependence was reported for (CdSe)ZnS [12] and CdTe [13], [14], [15]. From these previous studies, it is plausible to consider that the NC surface reaction with H⁺ or OH⁻ influences the surface electronic states, resulting in the PL intensity change. Therefore, the surface coating is required to prevent this surface reaction.

The NC surface coating is also expected to form the linkage between the NC and the biological molecule. Intensive studies have focused on investigating the linkage ability of the coating materials such as thiol [16], [17], [18], polyethylene glycol (PEG) [19] and polyacrylic acid (PAA) [6], [10], [20], [21], [22]. In particular, the coating methods using the PAA have widely been employed to combine with a biological substance such as a cell or a protein [6], [10], [22]. However, the NCs employed for this application were synthesized in TOP/TOPO. This synthetic method is attractive since the nanocrystalline size can be finely tuned, although this synthesis requires a high temperature reaction. In contrast, in an aqueous phase, the synthetic reaction, e.g. CdS [19], [23], [24], [25], [26] and CdTe [13], [27] occurs under mild conditions, e.g. at ambient temperature and pressure. Also, the size of the NCs prepared in an aqueous solution can be adjusted by the size-selective photoetching technique developed by our group. Thus, the PAA coating method using the NCs synthesized in an aqueous solution is required to be established for further applications. Moreover, clarifying the pH dependence on the PL intensity of the PAA coated NCs is inevitably important for application to the biological labeling.

In this paper, we demonstrate the methods to modify CdS NCs with cationic surfactant and PAA, and the pH dependence on the PL intensity of the PAA coated NCs. As shown in Scheme 1, we attempted to prepare the PAA coated NCs by two consecutive steps. The NCs were prepared in aqueous solution, and subsequently modified by cationic surfactant, inducing a phase transfer reaction. The transferred NCs were finally coated by the PAA, and dispersed in an aqueous solution.