Exploring the Use of a Lipopeptide in Dipalmitoylphosphatidylcholine Monolayers for Enhanced Detection of Glyphosate in Aqueous Environments

The growing reliance on pesticides for pest management in agriculture highlights the need for new analytical methods to detect these substances in food and water. Our research introduces a SPRWG-(C18H37) lipopeptide (LP) as a functional analog of acetylcholinesterase (AChE) for glyphosate detection in environmental samples using phosphatidylcholine (PC) monolayers. This LP, containing hydrophilic amino acids linked to an 18-carbon aliphatic chain, alters lipid assembly properties, leading to a more flexible system. Changes included reduced molecular area and peak pressure in Langmuir adsorption isotherms. Small angle X-ray scattering (SAXS) and atomic force microscopy (AFM) analyses provided insights into the LP’s structural organization within the membrane and its interaction with glyphosate (PNG). Structural and geometric parameters, as derived from in silico molecular dynamics simulations (MD), substantiated the impact of LP on the monolayer structure and the interaction with PNG. Notably, the presence of the LP and glyphosate increased charge transfer resistance, indicating strong adherence of the monolayer to the indium tin oxide (ITO) surface and effective pesticide interaction. A calibration curve for glyphosate concentration adjustment revealed a detection limit (LOD) of 24 nmol L–1, showcasing the high sensitivity of this electrochemical biosensor. This LOD is significantly lower than that of a similar colorimetric biosensor in aqueous media with a detection limit of approximately 0.3 μmol L–1. Such an improvement in sensitivity likely stems from adding a polar residue to the amino acid chain of the LP.


Equation S1.
Where:  is the peak current,  is the number of electrons transferred during oxidation or reduction, A is the electroactive area of the electrode (cm 2 ), D is the diffusion coefficient (cm 2 s -1 ), C is the concentration of the electroactive species (molcm -3 ), and V is the scan rate (Vs -1 ).Through this equation, we can determine the electroactive area of the electrode using a solution of 5 mmol L -1 K4Fe(CN)6/K3Fe(CN)6 in 0.1 mol L -1 KCl as a probe. .

Figure S1. Different views illustrate the steps in constructing the LP/PC monolayer biosensor.
Panel (A) shows the procedure from a diagonal perspective, while panel (B) provides a frontal view.In both views, the ITO surface is used as a substrate, and the LP/PC monolayer is transferred onto it using the Langmuir-Schaefer technique.Subsequently, PNG is applied to the biosensor surface, followed by electrochemical analysis.Equation S1: Here:   is the peak current,  is the number of electrons transferred during oxidation or reduction, A is the electroactive area of the electrode (cm 2 ), D is the diffusion coefficient (cm 2 s -1 ), C is the concentration of the electroactive species (molcm -3 ) and V is the scan rate (Vs -1 ).Through this equation, we can determine the electroactive area of the electrode using a solution of 5 mmol L -1 K4Fe(CN)6/K3Fe(CN)6 in 0.1 mol L -1 KCl as a probe.

Figure S3 .
Molecular models (top and side views) of the simulated lipid monolayers.(A) [LP/PC]=0 and (B) [LP/LC] = 0.30.Carbon atoms of PC aliphatic tail are colored in grey, oxygen and nitrogen atoms from PC polar head in red and blue, respectively, and LP in green.The yellow square (in top views) represents the unit cell containing the actual atoms, surrounded by image atoms from the periodic cell extensions.Figure S4.Compressibility modulus for different LP to PC molar ratios LP/PC, ranging from 0.05 to 1.00..

Figure S4 .
Figure S4.Compressibility modulus for different LP to PC molar ratios LP/PC, ranging from 0.05 to 1.00.

Figure S6 .
Figure S6.Surface pressure isotherm as a function of A -3/2 at different ratios for [LP/PC]=1.00for different PNG concentrations.

Figure S7 .
Figure S7.The surface potential of LP/PC monolayers in the presence of 15 µmol L -1 PNG.The graph demonstrates an increase in the SP value upon adding LP to the monolayer, indicating potential interactions between the pesticide and the monolayer.

Figure S8 .
Figure S8.Radial distribution function (g(r)) of PNG along the MD trajectories of [LP/PC]=0 (red curve) and (B) [LP/LC]=0.30(blue curve) systems.The distance was calculated concerning the polar group (head) of PC or the center of mass of serine residue in LP, respectively.
, the first term refers to the angular coefficient of the lines obtained in the graph of peak current versus the square root of the scan rate.Where:  = 1,  = 5 × 10 −6  −3 , D is the diffusion coefficient of potassium ferricyanide, equal to 6.39 × 10 −6  2  −1 .

Figure S11 .
Figure S11.Electrochemical analysis illustrating the modification of ITO with a 0.30 molar ratio LP/PC monolayer in the presence of interfering pesticides.The Nyquist plot shows the response of unmodified ITO (black line), ITO modified with the monolayer (red line), ITO modified with the monolayer in the presence of Carbaryl (pink line), and ITO modified with the monolayer in the presence of the interferent Malathion (green Line).