UC Berkeley

Direct observation of temperature distributions inside single living cells is a challenging yet fundamental research topic. In this work, semiconductor nanocrystalline quantum dots (QDs) were introduced as tiny temperature markers and three distinctive achievements have been accomplished: (1) optical property characterizations of QDs with respect to temperature and chemical changes inside single living cells; (2) temperature characterizations of micro heaters using QDs; and (3) imaging of thermogenesis inside single living cells due to chemical and thermal stresses.

The spectral shift of a single CdSe/ZnS core shell QD coated with a layer of organic polymer, and conjugated with streptavidin, was successfully characterized as 0.1 nm/°C around room-temperature. Empirical relations and statistical analyses concluded that about 1200 QDs are required to achieve 1°C statistical measurement precision in optical readouts. The proof-of-concept experiment utilized QDs to characterize the temperature distributions of a MEMS heater. Both experimental and simulation results showed good consistency and a 267 nm spatial resolution has been achieved.

QDs with central emission wavelength at ~655 nm were delivered to living cells by endocytosis and distributed in the form of vesicles in the cytoplasm. Their temperature-dependent spectral shift was measured and characterized with a linear relationship at 0.06 nm/°C. It was also observed that pH variations due to chemical changes have little effects on the spectra shift of QDs.

Thermogenesis of single living cell with respect to external chemical and temperature stresses were investigated. NIH/3T3 cells showed a measureable temperature increase with respect to high concentration of calcium influx. A cold-shock assay was conducted in a 15 min experiment, whereby the ambient temperature of cell culture was precipitously lowered from 37 to 20oC. It is found that single living cells exhibited higher average temperature than the environmental stimulations during the cold-shock experiments. This implies possible heat generation during cold exposure, presumably resulting from the complicated biochemical reaction networks, as a self-defense mechanism. Moreover, the observations of temperature distribution are in sharp contrast between live and dead cells, in both calcium influx and cold shock experiments. It suggests that heat generation is indeed a characteristic of living mammalian cells, with dead intact cells as controls. A negative control experiment using heat-shock was performed and results showed the temperature of NIH/3T3 fibroblasts trace closely with the external environmental temperature in a heating process from 35 to 50oC. Furthermore, intracellular temperature difference has been observed and recorded as large as 5oC during cold shock experiments.

These experiments demonstrate that quantum dots are capable of mapping intracellular temperature dynamically in single living cells with statistically improved spatial resolution and sensitivity. With no complex experimental setup, this technique could be widely applicable for the thermodynamic studies of single living cells.