June 03, 2019

Using Colorimetric Sensor Arrays for Cannabinoid Detection

A novel method for analyte detection shows promising applications for both cannabis and hemp markets.



Colorimetric analysis has been a popular technique within the field of chemistry, specifically for measuring the concentration of compounds within a solution. The concept is simple: a light source at a specific wavelength passes through a sample of interest onto a calibrated detector. The amount of transmitted light detected by the sensor is then used to determine the concentration of the substance.

Through this process, compounds, enzymes, and other analytes (substances) can be detected with a high degree of accuracy. 

“These sensors work differently than the technique described above. They change color based on recognizing a particular analyte - for example, an abused substance, some environmental contaminant, or a pesticide,” says Dr. Andrea Holmes, Professor of Chemistry at Doane University in Nebraska. “These arrays are a specific combination of color sensors with a very specific pattern. It’s almost like a unique fingerprint.”

Technological Innovation in Colorimetry

Until recently, identification of analytes had been constrained to a laboratory setting, requiring expensive equipment and trained staff to prepare and interpret the chemical analyses. As a result, field applications needing on-demand chemical testing – such as the detection of explosives – must rely on heavy instruments with power requirements and extension cords.

Along with her team of researchers, Holmes is looking to change that. She has been working on developing portable, deployable solutions within the colorimetric array space – solutions that have implications for the hemp and cannabis industries as well.

“I decided to pursue a technology that everybody knows - inkjet printing. You can go to any office supply store and buy one,” says Holmes. “You can make a very specific formulation so that the consistency, the thickness, and the viscosity is just like the ink that you use in your printer cartridge. Once we achieved that, we were able to inject our sensors into empty ink cartridges, put them in the printer, and print the arrays.”

While a seemingly simple process, “it took a lot of manpower, a lot of hours, and a lot of money to get it down to that simple technology.  Now, the arrays can be easily printed on a piece of paper and depending on the application, you get to custom-pick what kinds of sensors are deposited on the paper,” says Holmes. 

Applications for Cannabis and Hemp

In addition to identifying the type of substance, colorimetric arrays can also be used to determine the concentration of a particular element.

This capability could prove particularly useful when testing for concentrations of various cannabinoids in medical and recreational marijuana plants, or ensuring that hemp crops meet Farm Bill regulations.

“Wouldn’t it be great if the farmer could test along the way?” asks Holmes. She continues, “instead of being told by the Department of Agriculture oops no – you have to destroy your entire crop,” which is required if the crop tests above the maximum 0.3% threshold for THC. The financial implications within the hemp industry are substantial: hemp farmers would be able to test for THC concentration in the field, thus being able to determine when to harvest without risking the loss of crops.

Cannabinoids and Contaminants

To further this research within the cannabis and hemp industries, Holmes took a sabbatical year to co-found Precision Plant Molecules, a Colorado-based hemp extraction, and CBD distillate company. There she serves as Chief Growth Officer, focusing on the commercial development of 100% THC-free cannabis products.

“My whole motivation of going to Denver was to investigate whether or not this technology could be used to make a customized cannabis detection system,” says Holmes. “Testing of cannabis started late last year. Preliminary results are really promising, but it’s a project that has to be further investigated. If we continue to have the same success with cannabis as we have with other analytes, then we should be able to identify and quantify the different types of cannabinoids. Then, we can take it a step further and analyze all of the different pesticides that are regulated to identify those types of contaminants [within the crop].”

Biological contaminants, such as fungus, are also top-of-mind when considering contaminants of any cannabis crop. However, these colorimetric arrays are calibrated explicitly for chemical detection and currently cannot detect larger biological molecules or proteins.

Extraction Safety

Another concern paramount in the extraction industry is the amount of harmful chemical exposure of organic solvents to workers throughout the workday.

“A lot of cannabis extraction processes use hydrocarbon solvents, like butane, pentane, and ethanol. The people that are working in these types of environments are exposed these different types of organic solvents,” says Holmes. “Wouldn’t it be great if we could have a sensor that employee could wear? Developing sensors that can prevent any kind of hazardous exposure for extraction companies is something that’s very close to my heart.”

Future research in colorimetric arrays

In addition to further testing with cannabinoids, Dr. Holmes is working on developing a smartphone app to be used for field testing.

“Everybody I know of, even my parents, all know how to use the camera on their phone. We’re working on figuring out a technology where your smartphone captures the image of the colorimetric array, with software that identifies the analyte. I have a basic code for the software, but it needs to be developed further with more funding,” says Holmes.