Team:NTHU Taiwan/Quantum Dots

Quantum dots


What are quantum dots?

Quantum dots are nanoparticles with size range from few to tens of nanometer. This particle can be used in display panel to create even better resolution, high dynamic range colors. Quantum dots are well researched in engineering field. For instance, material scientist focus on the nucleation process and its commercial potential, biomedical engineer study the possibility about drug delivery system, bio-imaging. In synthetic biology, plenty of research indicate that certain microorganism are capable of producing quantum dots.


Property and Mechanism

Quantum dots are one kind of semiconductor ingredient, when we discuss about semiconductor, energy gap is the most important character that can decide the distribution of both electron and electron holes. The energy gap indicate energy emission when it jump from high energy level to a lower energy level. The energy gap of CdS which is the quantum dots we focus on is 2.53eV to 2.62eV. In the case that we excite quantum dots with wavelength equals to 365nm, due to this property, we are able to detect a emission. This emission is in the range of visible light for CdS case.


The problem we notice

In present, there are a lot of ways to synthesis quantum dots, but most of them contains a deep flaw that would harm the world and whoever live in it. For instance, Sol-Gel method require plenty of chemical solution, Solvothermal method need to be conducted in both high temperature and high pressure under nitrogen environment. Photochemistry Method even listed Cobalt 60 as one of the ingredients. These are how quantum dots are made currently and they cause industrial pollution that nobody wants.



We propose “BioSQUAD” as our solution. There are five magnificent members in this squad. Each of them owns their capabilities to solve this manufacturing problem. In literally, BioSQUAD means BioSynthesized Quantum Dots. We design our project by starting with increase the highest amount of cadmium ion that each E.coli can uptake. Next we find lot of genes and chose those can optimize fluorescence intensity. For the hardware, we design a bioreactor which is capable of automatically monitoring quantum dots' biosynthesis process.


Experiment Design: Inter vs Extra

From paper[1], We know there might be two pathways for biosynthesis of CdS. One is extracellular ,which means Quantum Dots might be synthesized by protein which contact outside the E.coli membrane and release to the culture buffer. The other pathway is intercellular, which means QD will be synthesized by protein which inside the E.coli, therefore Quantum Dots will stay inside of the bacteria[1].

Intercellular experiment :

Different kinds of bacteria might use different pathway to produce QD, we want to check how JM109 produce Quantum Dots but the pellet's fluorescence doesn't bright enough to clarify. We design another experiment to verify it. We add cadmium ions to bacteria for certain period of time, and then eliminate cadmium when we conduct the QD experiment. By this method, bacteria can only use cadmium, which is remain inside through mnth protein. Steps are shown below.

1. Culture bacteria in LB containing cadmium overnight

2. Change new M9 medium which has no cadmium inside to culture bacteria

3. Add Cysteine

4. Result : Observe the fluorescence pattern

Extracellular experiment:

At the early stage of experiment, we noticed if LB medium existed while being excited by Ultraviolet light wavelength = 365nm. A significant blue light emission might have been detected. This is due to the complexity of ingredients inside LB solution.The short wavelength emission was too intense for us to measure the curve of quantum dots. We thought of a way that washing the bacteria by M9 solution twice to reduced the amount of LB solution last in the solution.

1. E. coli grow for 12-20 hours (O.D. 600 value reaches 0.3)

2. Change new M9 medium which has no cadmium inside to culture bacteria

3. Washed with M9 solution twice

4. Detect the fluorescence spectrum after 1 hour, 2hour, 3hour...



Fig1. The tube contains M9 solution, E.coli, cysteine, cadmium ions. Changing the concentration of cysteine, we will see the different visible light emission excited by UV light (365nm).

Fluorescence spectrum detection result

Fig2-1. The fluorescence spectrum of JM109(Control Group)

Fig2-2. The fluorescence spectrum of JM109 with cysteine(concentration=0.1mM)

Fig2-3. The fluorescence spectrum of JM109 with Cadmium ion(concentration=0.1mM)

Fig2-4. The fluorescence spectrum of JM109 with cysteine/Cadmium ion(concentration=0.1mM)

The tube contains M9 solution, E.coli, cysteine, cadmium ions. Changing the concentration of cysteine, we will see the different visible light emission excited by UV light (365nm).

Application & target audience

There are many applications of quantum dots in plenty of fields, including Material Science, Biomedical Engineering, and even Physics. To name but a few:

(1) Sewage Detection

(2) Drug Delivery System

(3) Screen Panel

With this numerous of potential usage, we focus on something is easy to use, able to aware the public. Here, we composed a system that can detect the waste water concentration by simply detect the fluorescence intensity of quantum dots before and after add the sewage solution.

By the Stern-Volmer Equation:


where F is fluorescence intensity, Ksv is the Stern-Volmer constant, [Q] is the concentration of heavy metal ions in sewage. With detection and calculation, a linear relationship in the right figure can derived from the left figure.

Fig3. Effect of [Hg] ions concentrations on fluorescence intensity of CdS QD measured by Kaur, J., Komal [4]. A linear relationship is shown. Inspiring us to create a system based on our current project to build a powerful detector for sewage.

With this notions, we are able to share our project to more people and have influence on the real world. Public will understand the danger of this invisible foe, thus impact authorities concerned with both manufacturing and environment. We respectfully expect successor whoever interest in this topic will put this idea into practice.


  1. Biosynthesis of CdS Quantum Dots Mediated by Volatile Sulfur Compounds Released by Antarctic Pseudomonas fragiFront. Microbiol., 13 August 2019
  2. Low-temperature biosynthesis of fluorescent semiconductor nanoparticles (CdS) by oxidative stress resistant Antarctic bacteria
  3. Synthesis of salt-stable fuorescent nanoparticles (quantum dots) by polyextremophile halophilic bacteria
  4. Kaur, J., Komal, Renu et al. Glutathione Modified Fluorescent CdS QDs Synthesized Using Environmentally Benign Pathway for Detection of Mercury Ions in Aqueous Phase.J Fluoresc 30, 773–785 (2020).