Eu(III) modifies the properties of poly(N-isopropylacrylamide)
Introduction
In recent years, the potential applications of poly(N-isopropylacrylamide) (PNIPAM) materials have been under intensive study as a result of their potential uses in biomedical and biotechnological fields such as controlled drug delivery [1], biosensors [2], chemical isolation [3], bioelectrocatalysis [4], and magnetically controlled electrochemical reactions [5]. However, to the best of our knowledge, most of the reported stimuli-responsive PNIPAM only respond to the temperature-induced transition, and few paid attention to the optical property of PNIPAM.
Until now, lanthanide complexes were often used as probes and labels for the direct determination of organic analytes and nucleic acids in the immunodiagnostic assays [6], [7]. For example, Eu(III) was chosen as a probe due to the well-documented sensitivity of its fluorescence [8]. It is well known that, as a result of introducing carbonyl group into the chains, PNIPAM can coordinate with some metallic ions, for instance, rare earth ions. In this paper, the effects of Eu(III) on the electron cloud density, phase transition temperature and energy transfer of PNIPAM were investigated by X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC) and fluorescence spectroscopy. It was found that PNIPAM could coordinate with Eu(III) and form the complexes of PNIPAM–Eu(III). The complexes have important temperature-responsive and fluorescence properties, in which the properties have been used for developing new applications in fluorescence and biomedical field, such as fluorescence probe [9], separations [10], drug delivery [1], etc. We hope that our research would provide the reference point for the new applications.
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Experimental
N-isopropylacrylamide (NIPAM) (Aldrich Chemical Co., Inc. 99%) was used without further purification. EuCl3 was prepared according to the method described in the literature [11]. α, α′-azobisisobutyronitrile (AIBN) was recrystallized from ethanol solution and dried in vacuum. All other reagents were of analytical grade.
A German Perkin-Elmer model Ls50B fluorescence spectrophotometer was used to measure fluorescence spectra of PNIPAM and PNIPAM–Eu(III), in which both the excitation and emission
XPS measurement
XPS has become a powerful tool for providing precise information concerning the core-level binding energies and linewidths, and the valence electronic structure of macromolecules [12]. The typical XPS spectra obtained from the PNIPAM and PNIPAM–Eu(III) complex are shown in Fig. 1. The average binding energy values of C1s, O1s, N1s and Eu3d of PNIPAM in the absence of Eu(III) were listed in Table 1. It was reported that the binding energy of N1s for C–N bonds is at about 400.4 eV and the binding
Conclusions
From the above experimental results, it can be concluded that the novel PNIPAM–Eu(III) complex formed by the interaction between PNIPAM and Eu(III) possesses not only the characteristic fluorescence of Eu(III), but also the temperature-responsive property of PNIPAM. The coordination between the oxygen and nitrogen of the acylamino group and Eu(III) cation makes the PNIPAM–Eu(III) complex to exhibit intensive characteristic fluorescence of Eu(III), and the maximum emission intensity of
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