Since 2014, strict regulations have governed the sale, transport, use, and waste disposal of uranyl acetate, a contrast agent in electron microscopy for over 40 years, due to its toxicity. These constraints have compelled many researchers to seek less toxic alternatives. Nowadays, numerous non-toxic and non-radioactive contrasting agents, such as UAR, Uranyless, and OTE, are readily available.
OTE, an alternative derived from Oolong tea extract, acts as a tannin, enabling the completion of etching through a lead citrate post-contrast process. Additionally, OTE, used as a beverage extract in Asia, falls between green tea (non-oxidized) and black tea (fully oxidized).
Our Oolong tea extract boasts a high tannin content, ensuring exceptional purity and maximum efficiency during contrasts.
To prepare the OTE solution, dissolve the OTE powder in 70°C water at a concentration of 0.2% using a magnetic stirrer. Complete dissolution, achieved within 15 to 30 minutes under agitation, requires filtering the solution at 0.45µ or preferably at 0.22µ. To maintain its quality, store the solution in a small Eppendorf tube or an AirLess Black bottle in the refrigerator, ensuring darkness at 4°C. Always allow the solution to return to room temperature before use.
When contrasting grids with OTE, immerse them in the solution for 15 to 20 minutes at room temperature in darkness. After contrast, drain the grids without rinsing or with a very brief wash (1 to 2 seconds). For post-staining, treat the samples with lead citrate according to Reynolds (Ref # 11300) for 1 to 2 minutes. Subsequently, drain, rinse briefly, dry the samples, and they are ready for observation.
The prepared aqueous OTE solution has a slightly acidic pH of 6, which can be adjusted by replacing water with a phosphate or cacodylate Na buffer.
It is important to note that OTE, being a food product, does not require a Material Safety Data Sheet (MSDS). If you wish to view the OTE preparation process, a tutorial video is available here: OTE Preparation Video.