Questions & explanations
1. Compare quantum dots and gold nanoparticles in signal amplification.
Quantum dots are tiny semiconductor particles that emit bright, stable fluorescence. Gold nanoparticles produce strong colors due to plasmon resonance and also enhance electrical signals. Quantum dots allow multiplexing because different sizes emit different colors. Gold nanoparticles are better for colorimetric and electrochemical signals. Both amplify the signal by increasing the number of detectable labels per binding event. Quantum dots need UV light to excite, while gold nanoparticles can be seen with simple light. Gold nanoparticles are often easier to conjugate to biomolecules. The choice depends on the detection method: fluorescence uses quantum dots, color/electrical uses gold.
2. Compare an optical biosensor and an electrochemical biosensor for detecting pesticides in terms of how they work.
An optical biosensor uses light to measure the reaction. For pesticides, it might use an enzyme that changes color when it reacts, or it uses a fluorescent dye that lights up when the pesticide binds. The amount of light absorbed or emitted is measured. An electrochemical biosensor, on the other hand, measures an electrical current or voltage. For example, an enzyme reaction that produces electrons can be measured by an electrode. Both are sensitive, but optical sensors can be read by eye or a small camera, while electrochemical sensors need a simple electronic reader. Electrochemical sensors often work better in dirty samples because light is not blocked by dirt.
3. The Mark-Houwink equation is often used for linear polymers. How would you expect the equation to change for branched polymers? Explain with an example.
Branched polymers have smaller intrinsic viscosity than linear polymers of the same molecular weight because they are more compact. The Mark-Houwink equation still holds, but the constants K and a are different. Typically, for branched polymers, 'a' is lower (e.g., 0.3-0.5) because viscosity is less sensitive to molecular weight. For instance, low-density polyethylene (LDPE), which is branched, has a lower 'a' than linear high-density polyethylene (HDPE). If you use linear polymer constants to estimate molecular weight of a branched sample, you will underestimate the true molecular weight. Therefore, separate calibration is needed for branched polymers.
4. What are trade-offs between using graphene versus quantum dots in biosensors?
Graphene is excellent for electrical sensors due to its high conductivity and large area, making it sensitive and fast. However, graphene does not emit light, so it is not suitable for fluorescence-based detection. Quantum dots, on the other hand, are great for optical sensing because they emit bright, stable fluorescence, but they are poor conductors. Quantum dots can also be toxic (e.g., cadmium-based) and require UV excitation, which can cause damage. Graphene is more biocompatible and can be used in wearable devices. The choice depends on the detection method: electrical applications favor graphene, while optical multiplexing favors quantum dots.
5. How do nanoparticles increase the sensitivity of biosensors?
Nanoparticles increase sensitivity in several ways. They have a large surface area that can carry many signaling molecules, amplifying the signal from a single binding event. For example, a gold nanoparticle can be covered with thousands of enzyme molecules that each produce a detectable product. Also, their unique optical properties mean a few nanoparticles can create a strong color or fluorescence. In electrical sensors, nanoparticles can enhance electron transfer, producing a larger current. They also allow detection of targets at extremely low concentrations, down to picomolar or femtomolar levels, which is crucial for early disease diagnosis.
6. Why is the Mark-Houwink equation useful in polymer science for quality control? Provide an example of how it is applied.
The Mark-Houwink equation helps determine polymer molecular weight quickly by measuring intrinsic viscosity, which is simpler than methods like light scattering. In quality control, a manufacturer can measure the viscosity of a polymer solution and use known K and a values to check if the molecular weight is within specification. For example, polyethylene glycol (PEG) has published Mark-Houwink constants. If a batch of PEG has an intrinsic viscosity lower than expected, it indicates lower molecular weight, which might mean the polymer is not suitable for its intended use. This allows fast, low-cost screening without expensive instruments.
7. Give an example of a biosensor that measures lactate during cell culture, and why it matters.
A lactate biosensor uses the enzyme lactate oxidase (LOx). It works similarly to the glucose sensor: LOx converts lactate to pyruvate and hydrogen peroxide, which is detected electrochemically. The current is proportional to lactate concentration. This matters because lactate is produced by cells when they use glucose without enough oxygen (anaerobic metabolism). High lactate indicates cells are stressed or not getting enough oxygen. By monitoring lactate, scientists can adjust the culture conditions, like increasing oxygen supply, to keep the cells healthy and producing the desired product (e.g., a vaccine or protein).
8. How do zero-order and first-order kinetics differ in their elimination rate?
In zero-order kinetics, the elimination rate is constant and does not change with the drug concentration. In first-order kinetics, the elimination rate is proportional to the drug concentration. This means that in first-order, a higher concentration leads to faster elimination, while in zero-order the rate stays the same. For example, if you double the dose of a drug with first-order kinetics, the elimination rate doubles. But for a zero-order drug, doubling the dose does not change the rate, so the drug stays in the body much longer. The half-life in zero-order depends on the dose, while in first-order it is constant.
9. What is a common reason that active compounds from herbal medicines often have low bioavailability?
Many herbal compounds are water-soluble but the body absorbs them poorly because they are too polar to cross gut membranes. They may also be quickly broken down by enzymes in the gut or liver before reaching the blood. For example, the antioxidant quercetin from onions has low bioavailability because of these issues. Some compounds are also pumped back into the gut by transporter proteins. The low bioavailability means that even if a herb has active compounds, their levels in the body might be too low to have an effect. Some traditional preparations use methods like fermentation or adding pepper to improve absorption.
10. Give an example of a biological process that follows Michaelis-Menten kinetics.
The breakdown of alcohol by the liver enzyme alcohol dehydrogenase follows Michaelis-Menten kinetics. At low alcohol concentrations, the rate of elimination increases with alcohol level. But at high alcohol levels, the enzyme becomes saturated and the elimination rate reaches a maximum (Vmax). Another example is the metabolism of the drug warfarin by the CYP450 enzymes in the liver. Many drug-drug interactions involve saturation of these enzymes. Additionally, the transport of glucose into cells by carrier proteins follows Michaelis-Menten kinetics. This behavior is common in many enzyme and transporter systems.
11. What is a biosensor used for bioprocess monitoring?
A bioprocess monitoring biosensor tracks parameters like glucose, lactate, pH, or oxygen in a fermentation or cell culture. It uses a biological part like an enzyme or microbial cell that reacts with the substance of interest. The reaction produces a signal (electrical or optical) that is measured in real time. For example, a glucose biosensor uses glucose oxidase enzyme that produces hydrogen peroxide, which is detected by an electrode. This allows scientists to adjust the process, like adding nutrients, to keep cells healthy. The sensor is often placed directly in the bioreactor or in a flow-through system.
12. Why is the resonant frequency important for mass-sensitive sensors?
The resonant frequency is the natural vibration frequency of the sensor when no mass is added. It is important because any added mass changes this frequency in a predictable way. By measuring the shift in resonant frequency, you can calculate the added mass very precisely. The sensor is most sensitive when operated at or near its resonant frequency because the vibration amplitude is largest, making frequency changes easier to detect. For example, a QCM works best at its resonant frequency around 5-10 MHz. Environmental factors like temperature can also shift resonant frequency, so they must be controlled.