The Allure of Diamonds Goes Beyond Gemstones

Yasuaki Einaga (Associate Professor, Department of Chemistry)

The object held by the tweezers in the photo lacks the "eternal sparkle" one might expect... (it's gray!). However, this is a genuine diamond. Its black color comes from being a plate-like structure composed of tiny, tightly packed diamond particles, each only a few microns in size. In fact, this diamond has the potential to be used as a "high-sensitivity sensor" capable of instantly and accurately measuring even trace concentrations of substances such as uric acid and glucose levels in blood and urine, or arsenic, metals, and endocrine disruptors in groundwater. This is the "diamond electrode," a new material that is currently attracting attention.

In our laboratory, we create these diamonds using a device called a microwave plasma CVD (Chemical Vapor Deposition) system. We use a carbon-containing gas or solution as the raw material; even alcohol will suffice. These materials are decomposed within a plasma generated by microwaves (Figure 2) and deposited onto a substrate as diamond.

Typically, the high-pressure/high-temperature (HPHT) synthesis method is used to create artificial diamonds. This process requires recreating conditions similar to those deep within the Earth—approximately 100,000 atmospheres and 2,000°C—which calls for equipment so large it can fill an entire room. In contrast, the CVD system in our laboratory occupies a space of only about 1.6 square meters. Moreover, if we turn it on in the morning, a "high-sensitivity diamond electrode sensor" is ready by evening. A major difference between this diamond and a gemstone diamond (aside from its size) is its ability to conduct electricity. During the fabrication process, mixing boron into the source gas (a process called doping) results in a conductive diamond. This "diamond electrode" possesses useful properties, such as high resistance to water electrolysis when a voltage is applied. We have discovered that by applying a specific voltage in water (or, in practice, in blood, urine, groundwater, etc.) and measuring the resulting current, we can accurately determine the concentrations of substances like uric acid, glucose, arsenic, metals, and endocrine disruptors.

We are now enjoying our research, confident that this fascinating material will one day play a significant role in medical and environmental analysis.