Compositions and methods for detecting the activity of an ion channel in a cell are described. The methods include providing a loading buffer solution to the cell, where the loading buffer includes a thallium ion indicator and optionally chloride ions, and providing a stimulus buffer that includes thallium ions to the cell. Providing the stimulus buffer can cause thallium ion influx into or efflux out of the cell through the ion channel. After providing the stimulus buffer, a change in at least one optical property of the thallium ion indicator is detected in response to thallium influx or efflux, thereby detecting the activity of the ion channel.