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Nanoparticle assemblies for temperature measurements

Polymeric molecules that expand and contract with temperature are used to connect assemblies of nanoparticles, exemplifying nanoscale devices that can be used as probes for measuring temperature in fluids.
12 June 2006, SPIE Newsroom. DOI: 10.1117/2.1200605.0154

We have developed a nanoscale thermometer made of a combination of inorganic and polymeric materials, that are used to build highly ordered nanoscale superstructures. These are about 30nm in diameter, respond to temperature differences of 20 to 60°C, and can measure localized temperatures in volumes only 200nm across.1

Localized thermal control is an important factor for circuitry fabricated using advanced lithographic techniques, because thermal and electronic stability are closely linked. Together with biological indicators, these devices can be used to treat certain diseases that require more precise diagnostics, such as in anti-cancer thermotherapies and genetic analysis. They are also useful in the optimization of nano/microfluidic devices and in the thermodynamic design of electronics. However, there is no suitable technique that measures localized temperatures sub-micron scales. Recently developed nanotech sensing devices—such as nanohybrids of bioconjugates, hybrid nanocolloids, and their lattices—are limited because the way they respond to certain external parameters, such as temperature, is not properly controlled. This is because most current nanostructures are not flexible when they take the form of superstructures.2–5 To overcome this limitation, soft materials must be combined with polymers to create dynamic nanomaterial superstructures that adapt gradually. Such systems can also find technological application as novel optical devices.

Nanoscale thermometers are composed of a 20nm gold nanoparticle core with polymeric coils joined to its surface. This polymer, poly(ethyleneglycol) or PEG, has two different end functional groups—t-butoxycarbonyl (t-BOC) and N-hydroxysulfosuccinimide (NHS)—that attach to different nanoparticles. The CdTe semiconductor nanoparticles that photoluminesce are then covalently linked to the end of each polymer (see Figure 1). These polymers keep the distance between different types of nanoparticles within a few nanometers, while contracting and relaxing depending on external temperatures and so bringing the nanoparticles closer together or farther apart. These distance changes modify the interaction energy between the gold and semiconductor nanoparticles (i.e., between the surface plasmon of the gold nanoparticle and exciton of the semiconductor nanoparticles), thus inducing the superstructural change.

When this superstructure is irradiated with laser light, CdTe nanoparticles are efficiently excited at 568nm. The exciton energy of these nanoparticles resonates with that of the gold nanoparticle plasmon at 549nm. This resonance condition enhances photoluminescence, rendering it very sensitive to the interparticle distance. We varied the temperature of the CdTe-PEG-Au superstructure between 20–60°C using a heating/cooling circulator. This led to a temperature increase and, in turn, to an expansion of the PEG molecules and an increase in their diameter.6 By measuring the amount of light the nanothermometers emit in response to a laser scan, temperature changes as low as 1 or 2°C can be detected (see Figure 1). Non-PEG bonded systems did not reveal any temperature dependence. It is important to note that this process is completely reversible, with less than 10% photodegradation with every temperature cycle (1500s, see Figure 1). The lasers themselves do not significantly heat the nanothermometers. Theoretical calculations show that the presence of gold nanoparticles can either increase or decrease the fluorescence of CdTe depending on the resonance conditions. For example, the shift of a plasmon peak towards the exciton resonance induces an enhancement of fluorescence, while a shift away from the exciton energy results in a decrease of fluorescence intensity.

Figure 1. (Left) Suggested schematic of nanohybrid superstructures. (Right) Photoluminescent intensity variation of PEG-tethered Au and CdTe nanoparticles with temperature: a) temperature, b) photoluminescent intensity, and c) calculated photon-field enhancement factor of CdTe nanoparticles as a function of time.

In summary, nanohybrids of polymeric and inorganic nanoparticles have been fabricated as nanoscale sensing devices. These devices are usable long-term and have a strong reversible sensitivity to external temperatures. The synergistic effect of dynamic structures of nanohybrids and plasmon-exciton interactions produces a highly sensitive optical output. Furthermore, these devices are examples of a new family of sensing and optoelectronic devices that can be used to detect nanoenvironmental parameters such as pH levels or ionic strength, simply by a change in polymer.

Nicholas Kotov
Chemical Engineering, University of Michigan at Ann Arbor
Ann Arbor, MI
Nicholas Kotov graduated from Moscow State University in 1990, and did his postdoctoral training at Syracuse University, where he made the first nanoparticle assemblies. He has been an invited speaker at several SPIE meetings devoted to biological applications of nanotechnology and nanoscale optics.
Alexander Govorov
Physics Department, University of Ohio at Athens
Athens, OH