Have you ever wondered if your plants were dry and dehydrated, or if you’re not watering them enough? Farmers and green-fingered enthusiasts alike may soon have a way to find this out in real-time. 

Over the past decade, researchers have been working on sensors to detect a wide range of chemical compounds, and a critical bottleneck has been developing sensors that can be used within living biological systems. This is all set to change with new sensors by the Singapore-MIT Alliance for Research and Technology (SMART) that can detect pH changes in living plants — an indicator of drought stress in plants — and enable the timely detection and management of drought stress before it leads to irreversible yield loss.

Researchers from the Disruptive and Sustainable Technologies for Agricultural Precision (DiSTAP) interdisciplinary research group of SMART, MIT’s research enterprise in Singapore, in collaboration with Temasek Life Sciences Laboratory and MIT, have pioneered the world’s first covalent organic framework (COF) sensors integrated within silk fibroin (SF) microneedles for in-planta detection of physiological pH changes. This advanced technology can detect a reduction in acidity in plant xylem tissues, providing early warning of drought stress in plants up to 48 hours before traditional methods.

Drought — or a lack of water — is a significant stressor that leads to lower yield by affecting key plant metabolic pathways, reducing leaf size, stem extension, and root proliferation. If prolonged, it can eventually cause plants to become discolored, wilt, and die. As agricultural challenges — including those posed by climate change, rising costs, and lack of land space — continue to escalate and adversely affect crop production and yield, farmers are often unable to implement proactive measures or pre-symptomatic diagnosis for early and timely intervention. This underscores the need for improved sensor integration that can facilitate in-vivo assessments and timely interventions in agricultural practices.

“This type of sensor can be easily attached to the plant and queried with simple instrumentation. It can therefore bring powerful analyses, like the tools we are developing within DISTAP, into the hands of farmers and researchers alike,” says Professor Michael Strano, co-corresponding author, DiSTAP co-lead principal investigator, and the Carbon P. Dubbs Professor of Chemical Engineering at MIT.

SMART’s breakthrough addresses a long-standing challenge for COF-based sensors, which were — until now — unable to interact with biological tissues. COFs are networks of organic molecules or polymers — which contain carbon atoms bonded to elements like hydrogen, oxygen, or nitrogen — arranged into consistent, crystal-like structures, which change color according to different pH levels. As drought stress can be detected through pH level changes in plant tissues, this novel COF-based sensor allows early detection of drought stress in plants through real-time measuring of pH levels in plant xylem tissues. This method could help farmers optimize crop production and yield amid evolving climate patterns and environmental conditions.

“The COF-silk sensors provide an example of new tools that are required to make agriculture more precise in a world that strives to increase global food security under the challenges imposed by climate change, limited resources, and the need to reduce the carbon footprint. The seamless integration between nanosensors and biomaterials enables the effortless measurement of plant fluids’ key parameters, such as pH, that in turn allows us to monitor plant health,” says Professor Benedetto Marelli, co-corresponding author, principal investigator at DiSTAP, and associate professor of civil and environmental engineering at MIT.

In an open-access paper titled, “Chromatic Covalent Organic Frameworks Enabling In-Vivo Chemical Tomography” recently published in Nature Communications, DiSTAP researchers documented their groundbreaking work, which demonstrated the real-time detection of pH changes in plant tissues. Significantly, this method allows in-vivo 3D mapping of pH levels in plant tissues using only a smartphone camera, offering a minimally invasive approach to exploring previously inaccessible environments compared to slower and more destructive traditional optical methods.

DiSTAP researchers designed and synthesized four COF compounds that showcase tunable acid chromism — color changes associated with changing pH levels — with SF microneedles coated with a layer of COF film made of these compounds. In turn, the transparency of SF microneedles and COF film allows in-vivo observation and visualization of pH spatial distributions through changes in the pH-sensitive colors.

“Building on our previous work with biodegradable COF-SF films capable of sensing food spoilage, we’ve developed a method to detect pH changes in plant tissues. When used in plants, the COF compounds will transition from dark red to red as the pH increases in the xylem tissues, indicating that the plants are experiencing drought stress and require early intervention to prevent yield loss,” says Song Wang, research scientist at SMART DiSTAP and co-first author.

“SF microneedles are robust and can be designed to remain stable even when interfacing with biological tissues. They are also transparent, which allows multidimensional mapping in a minimally invasive manner. Paired with the COF films, farmers now have a precision tool to monitor plant health in real time and better address challenges like drought and improve crop resilience,” says Yangyang Han, senior postdoc at SMART DiSTAP and co-first author.

This study sets the foundation for future design and development for COF-SF microneedle-based tomographic chemical imaging of plants with COF-based sensors. Building on this research, DiSTAP researchers will work to advance this innovative technology beyond pH detection, with a focus on sensing a broad spectrum of biologically relevant analytes such as plant hormones and metabolites.

The research is conducted by SMART and supported by the National Research Foundation of Singapore under its Campus for Research Excellence And Technological Enterprise program.