Ciencias Exactas y Ciencias de la Salud

Access to clean and drinkable water is not yet a possibility in many parts of the world. In different regions or rural areas where common water sources are prone to contamination or places where the water treatment does not follow a strict and efficient protocol for disinfection and purification, different pathogens can end up being consumed by several people. Water in these and other places can be contaminated with relative ease, potentially facing from hundreds to thousands of deaths all around the world from diseases caused by unsafe water treatment and pathogenic bacteria. The bacteria Escherichia Coli O157:H7 is a highly pathogenic bacteria that causes a series of gastrointestinal complications and diseases. If left untreated, the consequences can be fatal. Because of this, the need for early detection of this pathogen in water supplies is of great importance. Aptamers are single-stranded chains of DNA or RNA whose sequence can be designed to fold into specific shapes. They can act as an alternative to antibodies for the recognition and binding to antigens and targets of interest. Aptamers have been studied for this potential, and have been successfully synthesized to bind to components in the outer cell membrane of pathogenic bacteria, allowing for a high selectivity. Dendrimers are poly-branched polymers that have the advantage of being able to grow to various generations, exponentially increasing the number of branches they have. Polyamidoamine dendrimers have been studied and researched for their potential in biomedical applications. The functional groups of the branches can be modified and have attached a functional group with the purpose of integrating them into sensing devices. The multiple branches of polyamidoamine dendrimers can then serve as multiple attachment sites for aptamer placement, increasing the amount of bacteria-capturing agents on a biosensing surface. Flow reciprocation is a concept that has been introduced in microfluidic devices, and consists in driving the fluid either actively or passively through a a microfluidic circuit in two directions. This can allow the fluid sample to return to the source or point of origin in the microfluidic device. This concept can be present in centrifugal microfluidics, in which passive ways of reciprocation have been implemented successfully. Pneumatic chambers have been used in order to achieve flow reciprocation by compressing air during the rotation of the device and expanding it during its deceleration. The compression of air is caused by the fluid entering the pneumatic chamber, and it returns to the source chamber due to the air expansion. Since this technique requires high rotational velocities, an alternative is to use flexible membranes as passive pumps that inflate upon rotation and relax during deceleration. These membranes allow for lower rotational speeds and the potential of ease of reciprocation. Flow reciprocation can then be used in biosensors to increase the interaction between the target and the biosensing surface. Then, this can be applied to increase the capturing of pathogenic bacteria by combining this concept with aptamers and dendrimers to achieve low limits of detection. Trial and error of microfluidic devices designs often leads to unused devices that end up discarded and in the trash, which translates into material going to waste. In order to optimize the design and save fabrication resources, computational simulations can be of use. By modeling the desired microfluidic device, one can change any relevant parameters before the actual fabrication of the final device. Lumped-element models are representations of a microfluidic circuit in which each component is governed by its own set of equations and global, system-wide variables are transmitted through ports, obeying Kirchhoff’s laws. The present work aims to combine all of these concepts into a centrifugal microfluidic device for the detection of Escherichia Coli O157:H7 pathogenic bacteria, utilizing aptamers attached to polyamidoamine dendrimers to have multiple capturing sites and using flow reciprocation to enhance the interaction between the bacteria and the aptamers for increased capturing. Fur- thermore, it aims to further develop a lumped-element model for the simulation of centrifugal microfluidic devices, incorporating a new component with elastic membranes for the purpose of flow reciprocation. A limit of detection of 8.137 × 103 CFU/mL was achieved.

In the last few decades, optical biosensors based on surface plasmon resonance (SPR) have attracted increasing attention as a label-free alternative for the detection of small traces of biological and chemical markers, for application ranging from drug discovery and medical diagnosis to food quality and national defense. These approaches exploit the high sensitivity of surface plasmons polaritons (SPPs) to variations in the refractive index of the medium surrounding a thin metal film, which is caused by adsorption of the analyte molecules in the metal-dielectric interface. However, nowadays the plasmonic biosensor platforms with best performance require of complex optical configurations and bulky instrumentation, which difficult its miniaturization capability and portability, limiting its integration with other bioanalytical tools. In this work, we propose a novel design based on a Mach-Zehnder interferometer (MZI), consisting on a gold layer with a subwavelength aperture surrounded by grooves, and a detection system based on intensity interrogation. Our proposed architecture contemplates independent control of the reference and sensing arms in a planar disposition, which allows the biosensor to operate in the region of maximum sensitivity for low-analyte concentration and avoid the requirement of using complex multilayer fabrication techniques. Through numerical simulations using the FDTD-method, we found that our platform performed satisfactorily compared to previously reported designs. Moreover, its miniaturization potential, small footprint, and simple illumination scheme make it an ideal candidate for use in integrated sensing systems, which can be further enhanced by multiplexing.

Ciencias Exactas y Ciencias de la Salud

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