Proving selectivity in MIP sensors: NIP controls, imprinting factor and interferent tests
A molecularly imprinted polymer (MIP) is often called a "plastic antibody": a polymer formed around a template molecule so that, once the template is removed, it keeps cavities that match the target in shape and chemistry. That is the promise, but a promise is not proof. When an MIP-coated electrode gives a signal, how do we know the signal comes from the target sitting in those cavities and not from something else? This article walks through the three checks analytical chemists use to answer that question.
Why a signal alone is not enough
An electrochemical MIP sensor reports binding as a change in current, often measured with techniques such as differential pulse voltammetry. Yarman and Scheller (2020) point out that this overall signal can include non-specific interactions with the non-imprinted parts of the surface and adsorption at the electrode, in addition to the specific binding in the cavities. Even well-made MIPs can show some cross-response towards chemically similar molecules (Herrera-Chacón et al., 2021). A sound validation therefore has to separate "binding in the cavities" from "sticking anywhere".
Step by step: the NIP control and the imprinting factor
The standard control is a non-imprinted polymer (NIP). It is prepared from the same components and under the same conditions as the MIP, but without the template, so it has the same chemistry and no shaped cavities (Akgönüllü et al., 2023). A typical comparison runs like this:
Prepare an MIP-modified electrode and an NIP-modified electrode in parallel, using the same procedure.
Apply the same template-removal and washing steps to both.
Measure the response of each electrode to the same series of target concentrations.
Divide the MIP response (or binding) by the NIP response: this ratio is the imprinting factor (IF).
An analogy helps. Press a key into soft clay and the imprint fits that key. A flat piece of the same clay tells you how much the key would stick anyway, simply because of the material. If the key sticks equally well to both, the imprint is doing nothing. An IF clearly above 1 means the cavities add recognition beyond the polymer's general stickiness; an IF close to 1 is a warning sign.
Interferent tests: look-alikes and real samples
A good IF shows that imprinting worked for the target, but it does not yet show what happens when other molecules are present. Two kinds of challenge matter. The first is structural analogues: molecules that resemble the target, such as drugs from the same therapeutic class, which might fit the cavities partly. The second is the sample matrix: substances commonly present in biological fluids such as serum. In an interference study, the target signal is measured with and without these substances, and a selective sensor shows little change.
Key takeaways
The NIP control: the same polymer without the template; it reveals the non-specific background signal.
The imprinting factor: the MIP response divided by the NIP response; values clearly above 1 indicate that the cavities matter.
Interferent tests: selectivity is only convincing after challenges with structural analogues and real sample components.
Selectivity checks at SAOnS Lab
At SAOnS Lab we develop MIP-based electrochemical sensors for drugs and biomarkers, and comparing imprinted and non-imprinted electrodes is part of how we check that a sensor really recognises its target. One example from our group is the electrochemical determination of the anti-cancer drug pazopanib with high selectivity and sensitivity using a molecularly imprinted polymer-modified glassy carbon electrode (Samancı et al., 2024).
Which interferents would you test first for your own analyte? If you are planning a selectivity study or would like to collaborate, contact SAOnS Lab through our website.
References
Yarman, A., & Scheller, F. W. (2020). How reliable is the electrochemical readout of MIP sensors? Sensors, 20(9), 2677. https://doi.org/10.3390/s20092677
Akgönüllü, S., Kılıç, S., Esen, C., & Denizli, A. (2023). Molecularly imprinted polymer-based sensors for protein detection. Polymers, 15(3), 629. https://doi.org/10.3390/polym15030629
Herrera-Chacón, A., Cetó, X., & del Valle, M. (2021). Molecularly imprinted polymers – towards electrochemical sensors and electronic tongues. Analytical and Bioanalytical Chemistry, 413(24), 6117–6140. https://doi.org/10.1007/s00216-021-03313-8
Samancı, S. N., Çetinkaya, A., Doufene, N., Bounoua, N., Kaya, S. I., Atıcı, E. B., & Özkan, S. A. (2024). Electrochemical determination of anti-cancer drug pazopanib with high selectivity and sensitivity using molecularly imprinted polymer-modified glassy carbon electrode. Journal of The Electrochemical Society, 171(3), 037504. https://doi.org/10.1149/1945-7111/ad2cba
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