Department of Chemical Engineering, University of Puerto Rico, Mayagüez Campus, PO Box 9000, Mayagüez, PR 00681-9000, United States.
J Colloid Interface Sci. 2010 Feb 15;342(2):540-9. doi: 10.1016/j.jcis.2009.10.041. Epub 2009 Oct 24.
The application of the response of magnetic nanoparticles to oscillating magnetic fields to probe transitions in colloidal state and structure of polymer-coated nanoparticles is demonstrated. Cobalt ferrite nanoparticles with narrow size distribution were prepared and shown to respond to oscillating magnetic fields through a Brownian relaxation mechanism, which is dependent on the mechanical coupling between the particle dipoles and the surrounding matrix. These nanoparticles were coated with covalently-attached poly(N-isopropylacrylamide) (pNIPAM) or poly(N-isopropylmethacrylamide) (pNIPMAM) through free radical polymerization. The temperature induced transitions of colloidal suspensions of these nanoparticles were studied through a combination of differential scanning calorimetry (DSC), dynamic light scattering (DLS), and AC susceptibility measurements. In the pNIPAM coated nanoparticles excellent agreement was found for a transition temperature of approximately 30 degrees C by all three methods, although the AC susceptibility measurements indicated aggregation which was not evident from the DLS results. Small-angle neutron scattering (SANS) results obtained for pNIPAM coated nanoparticles confirmed that aggregation indeed occurs above the lower critical transition temperature of pNIPAM. For the pNIPMAM coated nanoparticles DLS and AC susceptibility measurements indicated aggregation at a temperature of approximately 33-35 degrees C, much lower than the transition temperature peak at 40 degrees C observed by DSC. However, the transition observed by DSC is very broad, hence it is possible that aggregation begins to occur at temperatures lower than the peak, as indicated by the AC susceptibility and DLS results. These experiments and observations demonstrate the possibility of using AC susceptibility measurements to probe transitions in colloidal suspensions induced by external stimuli. Because magnetic measurements do not require optical transparency, these methods could be applied even in concentrated or opaque systems, in which light scattering techniques encounter technical problems.
应用磁性纳米粒子对振荡磁场的响应来探测胶体状态和聚合物包覆纳米粒子的结构转变。制备了具有窄粒径分布的四氧化三钴纳米粒子，并通过布朗松弛机制证明其对振荡磁场有响应，该机制依赖于粒子偶极子与周围基质之间的机械耦合。这些纳米粒子通过自由基聚合被共价接枝上聚（N-异丙基丙烯酰胺）（pNIPAM）或聚（N-异丙基甲基丙烯酰胺）（pNIPMAM）。通过差示扫描量热法（DSC）、动态光散射（DLS）和交流磁化率测量相结合，研究了这些纳米粒子胶体悬浮液的温度诱导转变。在 pNIPAM 包覆的纳米粒子中，所有三种方法都发现转变温度约为 30°C，尽管交流磁化率测量表明存在聚集，但 DLS 结果并未显示。对 pNIPAM 包覆纳米粒子进行的小角中子散射（SANS）结果证实，在 pNIPAM 的较低临界转变温度以上确实发生了聚集。对于 pNIPMAM 包覆的纳米粒子，DLS 和交流磁化率测量表明在约 33-35°C 的温度下发生聚集，远低于 DSC 观察到的 40°C 的转变温度峰值。然而，DSC 观察到的转变非常宽，因此，正如交流磁化率和 DLS 结果所示，聚集可能在低于峰值的温度下开始发生。这些实验和观察结果表明，使用交流磁化率测量来探测外部刺激诱导的胶体悬浮液转变是可能的。由于磁测量不需要光学透明性，因此这些方法甚至可以应用于浓度较高或不透明的系统中，在这些系统中，光散射技术会遇到技术问题。
