Water-soluble fluorescent conjugated polymer is a promising material which could be used as an optical platform in highly sensitive molecular sensors. In this paper, a simple label-free DNA sensor, which consisted of a poly(3-alkoxy-4-methylthiophene) and an aptamer, was used to detect L-argininamide (L-Arm). Due to the specific binding reaction between L-Arm and its aptamer, the proposed method can easily determinate the L-Arm through the recovery of fluorescence without any modification. Other ions or similar molecules had little effect on the detection. Moreover, there was a linear relationship between fluorescence intensity and the concentration of L-Arm. The detection limit of L-Arm was as low as 4.7？nM. 1. Introduction Recently, the use of water soluble fluorescent conjugated polymers (CPs) as either chemical or biological sensing elements has received wide interest [1–5]. CPs contain a large number of absorbing and delocalized molecular units, and the transfer of excitation energy along the whole backbone of the CP to the energy/electron acceptor results in an amplification of fluorescence signals. Therefore, CPs have been successfully employed in the detection of various substances including DNA, RNA, protein, metal ions, and even pH and temperature [6–9]. Aptamer, a kind of one single-stranded DNA or RNA sequences, can be synthesized with the systematic evolution of ligands by the exponential enrichment (SELEX) procedure [10, 11]. The unique properties of aptamer is that it can bind with a variety of targets ranging from small molecules, proteins, and even to cells at very high affinity. Unlike ELISA, immunobead assay, and western blotting, some unspecific binding will occur in such cases, aptamers provide decisive advantages. First, they are more resistant against denaturation and degradation. Second, their binding affinities and specificities can be manipulated easily and improved by rational design or by techniques of molecular evolution. Therefore, aptamer is widely utilized as recognition molecular for detecting a variety of targets ranging from small molecules, proteins, and even cells [12–17]. In this paper, we propose utilizing water-soluble fluorescent polythiophene derivative and aptamer for detecting L-Arm. On one hand, thiophene polymer can be easily prepared through oxidation of the corresponding monomers; on the other hand, thiophene polymer can detect, transduce, and possibly amplify chemical, biological, and physical changes into measurable optical or electrical signals with very high sensitivity [18–21]. 2. Materials and
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