New research demonstrates the viability of a cost-effective graphene-based biosensor to detect a range of biological molecules for use in medicine and food production.
Schematic representation of fabrication and biofunctionalization of the AJP graphene IDE. Image Credit: Iowa State University
Biosensors, devices that are capable of detecting chemical substances and biological molecules, have become important tools in a wide range of human activities and industries, including but not limited to agriculture, food analysis, medical testing and nanobiotechnology.
In particular, the detection of certain biomarkers is critical in diagnosing and treating disease, and for ensuring the quality of food products, thus minimizing waste.
As a result of the ongoing COVID-19 pandemic, the importance of biosensors has never been more abundantly clear. Technology is urgently needed to spot and identify respiratory viral diseases quickly and easily, often in remote and inaccessible locations.
A new paper¹ authored by Kshama Parate, from Iowa State University looks at cost-efficient biosensor prototypes that employ graphene, showcasing how these devices can be employed specifically in the areas of medicine and food quality control.
The author also assesses the production methods used to create these devices to answer how the disposable graphene-based biosensors of the future could deliver accurate and rapid results while remaining cost-effective.
What are the Limitations of Current Biosensors?
One of the hindrances to the wider adoption of such technology is the restrictive cost of such devices. Detecting biomarkers often relies on expensive lab techniques that must be performed by trained technicians, something that often delays the delivery of results.
The problem of expense in biosensors is compounded by the fact that many current devices feature precious metals as vital components, with gold particularly a popular choice for designers.
These metals are employed in the creation of electrodes by using high temperatures and vacuum conditions. The resulting electrodes are then patterned using UV light with excess metal carved away — something that also requires vacuum conditions and a high-power etching tool, commonly a 500 watt laser.
Clearly, creating these electrodes, which form the basis of the sensor element of biosensors, is not inexpensive and requires highly specialized equipment, but the process also creates a large amount of waste material.
There is a much cheaper alternative, however. Graphene, atom-thick sheets of carbon atoms arranged in a honeycomb-shaped pattern, could provide an affordable and efficient substitute for metals like gold.
Graphene has been heralded as a good choice for biosensors due to its electrical conductivity, porosity and extremely large surface area. All of these characteristics make it excellent at capturing and immobilizing biomarker molecules.
The question is; how should these graphene-based sensors be manufactured?
Making the Ideal Graphene-Based Biosensor
Graphene is inexpensive enough to obtain, easily carved away from graphite which itself is a highly abundant material that is cheaply obtained. But to maintain a low-cost device, it also needs to be relatively inexpensive to produce. The fact that many biosensors are single-use also means that viable production methods must be scalable and able to produce a large number of devices.
One of the most common ways to create graphene devices is to use low-yield chemical vapor deposition techniques (CVD), in which a substrate is exposed to precursors which leave behind a graphene deposit on its surface. Whilst coming with a clear track record of positive applications this method is simply too expensive for use in a disposable device.
Low-cost alternatives like screen and ink-jet printing do exist, but these don’t grant developers the control needed to create the complex geometry and the intricate patterns required for a high level of sensor performance.
Building upon previous work² Parate suggests a biosensor prototype created using aerosol jet printing (AJP) is used to create high-resolution interdigitated electrodes (IDEs) on flexible substrates. This has been used by the author and her University of Iowa colleagues to create graphene-based electrochemical sensor prototypes capable of detecting histamines and toxins in food much faster than standard laboratory tests.
AJP is an additive manufacturing method that deposits materials only where it is needed, meaning that there is no need to carve away excess material, thus cutting down on waste. The sensors created using this method are lightweight, portable, and straightforward to produce. The advantage of this is clear; food testing and medical investigation where it is most needed without the necessity of sending away samples and awaiting lab results.
Aerosol-jet printing was fundamental to the development of this sensor. Carbon nanomaterials like graphene have unique material properties such as high electrical conductivity, surface area, and biocompatibility that can significantly improve the performance of electrochemical sensors. But, since in-field electrochemical sensors are typically disposable, they need materials that are amenable to low-cost, high-throughput, and scalable manufacturing. Aerosol-jet printing gave us this.
Professor Carmen Gomes, Iowa State University
So far so good, but the big question is, how do these AJP created graphene biosensors perform?
In their 2020 study, Parate and her co-authors pitted their prototype sensor against two formidable opponents; a fish broth and a buffering solution (PBS).
The aim was to see how effective the AJP built graphene sensor was at spotting histamines. The researchers were also looking for was the effect that biological molecules from the fish had on the sensor’s efficacy.
“We found the graphene biosensor could detect histamine in PBS and fish broth over toxicologically relevant ranges of 6.25 to 100 parts per million (ppm) and 6.25 to 200 ppm, respectively, with similar detection limits of 2.52 ppm and 3.41 ppm, respectively,” Says Parate, explaining that these sensor results are significant, as histamine levels over 50 ppm in fish can cause severe allergic reactions and other adverse health effects such as food poisoning.
Not only this but they discovered other biological molecules had no impact on the sensor’s ability to detect histamines and the device could deliver results within 34 minutes.
For her latest study, Parate exchanged the fish broth for a minced fillet of fish and once again used the sensor to detect histamines. Again, the prototype sensor demonstrated a high degree of sensitivity.
This time the author went a step further, also using the device to test saliva for traces of cotinine — a metabolic form of nicotine. Cotinine has shown promise in a trace of smoke exposure because it sticks around for a prolonged period in the human body — particularly in bodily fluids.
In this latter test, the sensor was able to find extremely low traces of cotinine in the saliva of smokers and non-smokers alike. The device delivered results within 12 minutes and wasn’t affected by traces of nicotine and other similar chemical compounds found in tobacco.
Parate also tested the prototype sensor’s ability to resist mechanical stress, finding the biosensor was extremely robust. Perhaps the most appealing thing about the device was the fact that a unit could be produced at a cost of under 3 USD.
What does the Future Hold for Graphene-Based Biosensors?
There are many ways in which graphene-based sensors for detecting biological molecules could be of major benefit in food production and disease detection and treatment.
These benefits depend strongly on the production method used, its cost, scalability, and portability. With the right techniques, developers could create sensors that can be used in situ, delivering accurate results in just minutes. And Parate’s device certainly seems to meet these criteria.
The researcher suggests that the production methods used to create these prototypes could now be tested in the creation of biosensors to detect other molecules, including devices that can quickly identify foodborne pathogens and, importantly, the spike protein that is the tell-tale sign of COVID-19 infection.
Sources
1. Parate. K. W., [2021], ‘Graphene biosensors for healthcare and food spoilage monitoring,’ Iowa State University.
2. Gome. C., Parate. K. W., et al, [2020], ‘Aerosol-jet-printed graphene electrochemical histamine sensors for food safety monitoring,’ 2D Materials IOP Science,[https://iopscience.iop.org/article/10.1088/2053-1583/ab8919/meta]
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