This document discusses semiconductor quantum dots for electrochemical biosensors. It describes how biomolecules like glucose oxidase, hemoglobin, and myoglobin can be attached to quantum dots, either through covalent or non-covalent bonding, for use in various biosensors. Specific examples of quantum dot-based biosensors are provided that use (1) glucose oxidase and CdS quantum dots for glucose detection, (2) hemoglobin and CdS quantum dots for hydrogen peroxide detection, and (3) myoglobin attached to CdTe quantum dots supported on mesoporous carbon foam for hydrogen peroxide detection. The quantum dots facilitate direct electron transfer between the biomolecules and the electrode surface in these biosensors.

1. Semiconductor Quantum Dots for
Electrochemical Biosensors
Chunyan Wang , Bernard Knudsen , and Xueji Zhang
2017 October 31
Presenter: Kisan Chhetri
Student Id Number:201755415
Chonbuk National University
Department of BIN Convergence Technology Image Source @ http://www.popsci.com/scitech/article/2008-07/health-scare-day-quantum-dots
A Presentation for Partial Fulfillment of the Requirements for the Theory Class of Biosensor Nanomaterial under the supervision of Prof. Joong Hee Lee.
Chapter-10

2. 10.1 Introduction
10.2 Attachment of Biomolecules to Quantum Dots
10.3 Quantum Dot-Based Redox Proteins Biosensor
10.3.1 Glucose Oxidase – Quantum Dot-Based Glucose Biosensor
10.3.2 Hemoglobin – Quantum Dot-Based H2O2 Biosensor
10.3.3 Myoglobin – Quantum Dot-Based H2O2 Biosensor
10.3.4 Laccase – Quantum Dot-Based Ascorbic Acid Biosensor
10.3.5 Acetylcholinesterase – Quantum Dot-Based Inhibitor Biosensor
10.4 Quantum Dot-Based Electrochemical Biosensors of Proteins and DNA
10.5 Conclusions
References
Contents

3. For examples:
The quantum of charge is the charge of an electron. Electric
charge can only increase or decrease by discrete (distinct/
distinguishable) energy levels So, there is no half-charge
 A photon is a single quantum of light (or of any other form
of electromagnetic radiation), and can be referred to as a "light
quantum". Light and other electromagnetic energy is absorbed or
emitted in quanta or packets.
 Latin word for amount (quantus, which means “how great”) and, in modern
understanding, means the smallest possible discrete unit of any physical
property, such as energy or matter
Source: https://www.thoughtco.com
Before we jump into the course contents…………
.What does quantum means?

4. Introduction
figure : Cadmium sulfide (cds) QDs on cells high resolution
Source @- http://picturethis.pnl.gov/picturet.nsf/by+id/DRAE-8MJSF8
 Semiconductor QDs are semiconductor nanoparticles
made of group 12 – 16 (II-VI) ( Zn, Cd, Hg – O, S, Se ,
Te , Po) or 13 – 15 (III-V) (B, Al, Ga, In – N, P, As, Sb, Bi)
elements with diameter between 1 and 10 nm, so small
that their optical and electronic properties differ from
those of larger particles
 Many types of quantum dot will emit light of specific
frequencies if electricity or light is applied to them, and
these frequencies can be precisely tuned by changing the
QDs size and materials , giving rise to various
applications
 QDs are also sometimes referred to as artificial atoms, a
term that emphasizes that a quantum dot is a single
object with bound, discrete electronic states, as is the case
with naturally occurring atoms or molecules
“QDs are a central theme of nanotechnology”
Diameter 8.6 nm

5.  Larger QDs (radius of 4–6 nm, for example) emit longer wavelengths resulting in emission colors
such as orange or red, Smaller QDs (radius of 2–3 nm, for example) emit shorter wavelengths
resulting in colors like blue and green, although the specific colors and sizes vary depending
on the exact composition of the QD
 Because of their unique photophysical properties, such as high fluorescence, quantum yield
stability against photobleaching, and size - controlled luminescence properties, enable them to
be utilized as optical labels for bioanalysis
DOI:10.1039/c4cs00532e

6. When biomacromolecules are immobilized on the surface of photoexcited QDs,
this can promote direct electron transfer between the biomolecules and
electrode surface, which improve the performance of the biosensor
 The cathodic
photocurrent could be
formed by the ejection of
the CB electrons to an
electron acceptor in the
electrolyte solution,
followed by the supply of
electrons from the
electrode to neutralize
the VB holes
Zayats M. and et. al.
 By comparison, the anodic photocurrent can be
yielded by transferring the CB electrons to the
electrode with the transfer of electrons from an
electron donor in the solution to neutralize the VB
holes
CB
VBVB
CB

7. Attachment of Biomolecules to QDs
Biomolecules are
generally immobilized
on QDs by two
approaches
 For non-covalent attachment,
 Physical adsorption and entrapment
 For covalent attachment,
 QDs can be simply mixed with proteins
to make a protein – QD - modified
electrode
 These stabilized QDs can form strong
electrostatic interaction with proteins by
the anionic groups on the QDs and
cationic groups on the proteins
 some cross - linker agents (e.g., 1 -ethyl-
3-(3-dimethylaminopropyl) carbodiimide
hydrochloride/ N-hydroxysuccinimide) or
Schiff bases are usually used for covalent
binding to increase the stability or
durability
 The activity of the immobilized
biomolecules may be affected due to steric
hindrance by covalent binding
Non-Covalent
attachment
Covalent
attachment
Covalent Attachment
Direct Attachment
Fig: Stabilized QDs with thioglycolic acid ( TGA ) and
3 - mercaptopropionic acid to form an anionic surface
DOI: 10.3390/s6080925

8. QDs - Based Redox Proteins Biosensor
A. Glucose Oxidase – QD-
Based Glucose Biosensor
B. Hemoglobin – QD-Based
H2O2 Biosensor
C. Myoglobin – QD-Based
H2O2 Biosensor
D. Laccase – QD-Based
Ascorbic Acid Biosensor
E. Acetylcholinesterase – QD-
Based Inhibitor Biosensor
Semiconductor QDs incorporated with redox proteins for electrochemical biosensors to
determine glucose, H2O2 , ascorbic acid and inhibitors are discussed in this section
Table: List of QD - based redox protein biosensors
CdS . 10-3

9. A. Glucose Oxidase-QD-Based Glucose Biosensor
 Two different QD-modified glucose oxidase ( GOx ) electrodes (GOx/CdS/ PGE (plane graphite disk electrode) and
Nafion – CNT – CdTe – GOx/ GC (glassy carbon) were developed to determine glucose by Huang et al. and Liu et al.,
respectively
2e-
Fig. : Enhanced photoluminescence in the QDs-GOx
bioconjugates by electron transfer from glucose oxidation.
1.(GOx/CdS/ PGE)
Fig.: CVs of bare PGE (A), CdS/PGE (B)
and GOx/CdS/PGE (C)in a buffer solution
with pH 6.0
A CV of GOx/CdS/PGE (cycle C)
electrode shows a pair of well –
defined and reproducible peaks. In
contrast, bare PGE( cycle A) or a
CdS/PGE (cycle B) electrode which
is electrochemically silent
Therefore, the redox reactions of
GOx/CdS nanoparticles play an
important role in improving GOx
adsorption and facilitating electron
exchange between GOx and PGE
Detection Limit: 50 µM
 This mediator - free glucose
sensor shows linear glucose
concentration ranging from 0.5 to
11.1 µM with a sensitivity of 7.0
μA M-1 , which is much higher
than that of a mediator
polyelectrolyte glucose sensor
Scan rate: 200mVs−1
DOI:10.3740/MRSK.2009.19.1.044

10. All of these
results indicate
that the direct
electron
transfer of
GOx was
achieved
through the
incorporation
of QDs
It was probably due to
their unique chemical and
electrochemical
properties
It represents a new
electrochemical platform
that provides operational
access to a large group
of redox-active enzymes
for designing a variety of
QDs based bio-sensor
3. ( Nafion-CNT-CdTe-Gox/GC )
Fig.: CV of (a) Nafion-CdTe-GOD/GC, (b) Nafion-CNTs-GOD/GC, and
(c) Nafion-CNTs-CdTe-GOD/GC electrode in 0.05M PBS (pH 6.0) with
N2-saturated at a scan rate :50mVs−1
 Nafion-CNT-CdTe-GOx (Cycle c)– modified
electrode was much better than of Nafion-CdTe-
GOx/GC (Cycle a) and Nafion-CNT-GOx/GC
(Cycle b) and shows a pair of well – defined and
reproducible peaks, which indicates the presence
of synergistic effects in Nafion-CNT- CdTe -GOx
Nafion-CNT- CdTe -GOx
2. ( Nafion-CdTe-Gox/GC )
 No redox peaks were observed at the Nafion – CdTe/GC
(Cycle a) and the redox peaks at the Nafion – GOx/GC
(Cycle b) were very small. In contrast, there was a pair of
well - defined and quasi - reversible redox peaks at the
Nafion – CdTe – GOx/GC (Cycle c)
 Small peak potential separation ( Δ E = 26 mV), revealing a
fast electron transfer process
Fig.: CVs of (a) Nafion-CdTe/GC, (b) Nafion-GOx/GC
and (c) Nafion-CdTe-GOx/GC electrodes in 0.05M PBS
(pH 6.0) with N2 – saturated at a
Nafion – CdTe – GOx/GC
Linear Range : 0.7 mM
Sensitivity : 1.018 µAmM-1
scan rate of 50 mVs-1

11. Hemoglobin-Quantum Dot-Based H2O2 Biosensor
Nafion/Hb – CdS/ GP ( graphite electrode )
 Hb was immobilized to QDs by physically mixing the
Cd(NO3)2 with Hb and adding Na2S aqueous solution
 The QD - modified Hb solution was cast onto the
graphite electrode, followed by a Nafion film coating
to hold the Hb-QD film to the electrode
Fig. : Preparation process of the CdS–Hb /graphite electrode
Fig.: CV of a Nafion/Hb/GP; bNafion/CdS–
Hb/GP; c Nafion/CdS/GP in 0.1 M phosphate
buffered saline (PBS) pH 7 , Scan rate 300 mVs–1
 No redox peak was observed for (Cycle a) Nafion/CdS/GP
electrode and the (Cycle c) Nafion/Hb/GP electrode in the
same potential range but reversible redox peaks was seen
at (Cycle b) Nafion/CdS–Hb/GP which indicates that Hb
could exchange electrons directly with the graphite
electrode when it was modified with CdS
 The Nafion/Hb–CdS/GP electrode can be used as a
hydrogen peroxide (H2O2) biosensor because of its good
bioactivity
Hemoglobin
Hemoglobin is the iron
containing oxygen
transport metalloprotein in
the red blood cells of all
most all vertebrates

12. Fig.: CV of Nafion/CdS–Hb/GP in 0.1M pH 7.0 PBS containing
a 0, b 5, c 10, d 15 and e 20 µM H2O2. in pH 7.0 PBS
 Hb-CdTe-CaCO3@polyelectrolyte electrode
 Nafion/Hb – CdTe /GC electrode
Other examples of Hb-QDs
based H2O2 sensors are
(a) CaCO 3 microspheres were first
coated with polyelectrolyte (PE) layers
poly(sodium 4-
styrenesulfonate)/poly(allylamine
hydrochloride) ( PSS / PAH ) to produce
CaCO 3 at polyelectrolyte
microspheres
(b)The polyelectrolyte – protected
spheres were then used for the loading
of CdTe QDs to fabricate the CdTe –
CaCO 3 @ polyelectrolyte hybrid
material
(c)In the final step, the hybrid
material was used for the loading of
Hb. Protein was incorporated with QDs
by electrostatic interactions
Fig.: Fabrication of Hb-CdTe-CaCO3@polyelectrolyte electrode
a
b
c
This figure
indicates two
possible electron-
transfer modes,
directly and
indirectly between
one QD to the
protein redox
center
Hb
QD
 On addition of successive 5 μM H2O2 to the electrochemical cell, the
reduction peak current increases and the oxidation peak current
decreases, indicating a typical electrocatalytic reduction process of H2O2
 The current response and the concentration of H2O2 have a Linear range
from 5 × 10-7 to 3 × 10-4M and the Sensitivity is 1.9 μA μM-1, which is
much higher than that other reported Hb-immobilized H2O2 sensors
Linear Range :5 – 45 µM
Detection Limit :2.5 µM

13. Myoglobin-Quantum Dot-Based H2O2 Biosensor
Fig.: Fabrication process of Mb – QD – MCF hybrid material
 First, the TGA - modified CdTe QDs were
incorporated into functionalized amino groups MCFs
(mesopore cellular foam), based on the strongly
electrostatic interactions
 Second, positively charged Mb at pH 6.5 assembled
easily into a cavity of QD – MCF by electrostatic
interaction
 Finally, The Mb-QD-MCF suspension was cast onto
the GC electrode with PVA solution for encapsulation
Fig.: CV MCF/GC (a), Mb – MCF/GC (b), and Mb – QD –
MCF/ GC (c) electrodes in 0.1 M PBS solution at pH 7.0
 There was no redox peaks at the MCF/GC (Cycle a) electrode, indicating that MCFs were
not electroactive in the potential range
 The cyclic voltammogram of Mb – MCF/GC (Cycle b) showed relatively small reversible
redox peaks, which indicated the weak direct electron transfer between the immobilized
Mb and the electrode
 On the Mb – QD – MCF/GC (Cycle c) electrode remarkably large redox peaks were
observed, which indicated the greatly enhanced direct electron transfer between the Mb
and electrode after the protein immobilized in the QD–MCF matrix
QD – MCF
MFC
NH2 functionalized MCFs
Protonation
Scan rate: 200 mV s − 1
Fig.: Structure of Myoglobin
Myoglobin is an
iron and oxygen
binding protein
found in the
muscle tissue of
vertebrates in
general and in
almost all
mammals

14.  It demonstrates that under the same concentration of H2O2 , the current obtained
at the Mb – QD – MCF/GC electrode was much larger than that at the Mb –
MCF/GC electrode
 According to the linear range and sensitivity, it can be concluded that the Mb–
QD–MCF/GC electrode behaved better in terms of electrocatalytic performance
to H2O2 than the Mb–MCF/GC electrode
 When H2O2 was added to a pH 7.0 PBS solution, an obvious
increase of the reduction peak current at about − 0.346 V
was observed on the Mb – QD – MCF/GC electrode,
accompanied by the decrease of the oxidation peak current
shown in Figure
Fig.: CV of Mb – QD – MCF/GC
electrode in 0.1 M PBS solution at
pH 7.0 with (a) 0, (b) 2.5, (c) 5.0,
and (d) 7.5 μ M H2O2
Scan rate: 200 mV s-1
Fig.: Plots of the electrocatalytic current ( I )
versus H2 O2 concentration for Mb MCF/GC
(a) and Mb – QD – MCF/GC (b) electrodes in
0.1 M PBS at pH 7.0.
Electrode Linear Range
(μ M)
Sensitivities
(mA.cm-2M-1)
Mb–QD–MCF/GC 2.5 – 60 ✔ 9.0 ✔
Mb – MCF/GC 5.0 – 35 6.9
Mb–QD–MCF/GC
Mb–MCF/GC

15. Laccase – Quantum Dot - Based Ascorbic Acid Biosensor
 Laccase is copper
containing oxidase
enzymes found in
many plants, fungi,
and microorganism
Fig.: CV of (a) bare gold, (b) Cys/Au, (c) CdTe/Cys/Au, (d) Lc/Cys/Au, and
(e) Lc/CdTe/Cys/Au electrodes in BR buffer at pH 4.5
Scan rate: 100 mV s-1 e
e
 There were no redox peaks on the curve a, b, c, and d but a pair of well-
defined quasi-reversible peaks appeared on the Lc/CdTe/Cys/Au electrode
(curve e)
 After CdTe was introduced into this system the redox peaks on curve e were
caused by Lc and enhanced by CdTe QDs
 The peak-to-peak separation (ΔEp) = 172 Mv

reduction peak current (Ipc)
oxidation peak current (Ipa)
≈ 1
 All these electrochemical parameters indicate that the electrochemical
reaction of Lc immobilized on the electrode is a quasi-reversible process
DOI: 10.13140/2.1.3040.7689

16.  The modified electrode can be used as an AA biosensor with high reproducibility and stability) in
the 10 – 140 μ M concentration range, with a linear response curve and a detection limit of 1.4 μ M
 The electrocatalytic activity of the Lc/CdTe/Cys/Au is
0.47 mM, which implies that Lc immobilized on the Lc/
CdTe/Cys/Au electrode exhibited high catalytic activity
 It was also confirmed that Lc could keep its biological
activity when it had exchanged electrons with the
electrode
Fig. : Amperometric response of an Lc/CdTe/Cys/Au electrode
(a)and an Lc/Cys/Au electrode (b) with successive addition of 10
μM AA in buffer (pH 4.5) at an applied potential of 400 mV
 This study provides a novel way for the study of
electron transfer and catalytic mechanism of Lc,
and also broadens the application of QD based
biosensor
 Figure show that the catalytic currents
increase with the successive addition of AA

17. 2e-
Acetylcholine
Acetylcholinesterase – Quantum Dot - Based Inhibitor Biosensor
Fig.: Schematic diagram for biosensor fabrication and determination of organophosphate pesticide (Monocrotophos)
 AChE is an enzyme that catalyzes the breakdown of acetylcholine and of some other choline esters that function
as neurotransmitters
The GC
electrode was
activated and
covered by CMs
and GNPs
CM+ GNPs - covered GC
electrode was assembled
with carboxyl
group - functionalized
CdTe QDs
QDs bound to
AChE through
covalent binding
of Schiff base.
As a result of
Interactions
between amine
groups of AChE
and carboxyl
groups of QDs
AMPLIFIER

18.  Compared to the oxidation peak on the AChE – CdTe – GNP –
CM/GC electrode (curve d), the oxidation peak at 826 mV on the
AChE – CdTe – CM/GC electrode (curve e) and 689 mV on the AChE
– GNP – CM/ GC electrode (curve f) shifted positively and the peak
currents were much lower
 This suggested that the CdTe QDs can promote the electron transfer of
enzyme. However, their conductive properties and catalytic behavior
are not as good as GNPs.
Fig. : (A) CV of (a) bare GCE and (b) AChE-CdTe-GNPs-CM/GCE in pH 7.0 PBS; and (c) CdTe-GNPs-CM/GCE,
(d) AChE-CdTe-GNPs-CM/GCE (B) (d) AChE-CdTe-GNPs-CM/GCE, (e) AChE-CdTe-CM/GCE and (f) AChE-
GNPs-CM/GCE in pH 7.0 PBS containing 1.0mM ATCl.
 The combination of GNPs and
CdTe QDs further improved the
electronic transport, which may
be due to the combine effects of
CdTe – GNP film
 This sensor was also used to
detect the inhibition of
monocrotophos, which is involved
in the irreversible inhibition on
AChE
 The inhibition of monocrotophos
was proportional to its
concentration in two ranges, from
1 to 1000 ng ml−1 and from 2 to 15
μ g ml−1 with a detection limit of
0.3 ng ml-1

19. Quantum Dot - Based Electrochemical Biosensors of Proteins and DNA
 QDs as the label for the electrochemical detection of DNA and protein have been reported by Wang et al. since
2002
 The detection system is based on the stripping voltammetric detection of dissolved nanoparticles
 For example, CdS nanoparticles conjugated with DNA were used for the detection of DNA hybridization
Preparation of CdS
QDs
Preparation of CdS–
DNA conjugate
Preparation of oligonucleotide-
coated magnetic beads
Particle enlargement and
magnetic ‘collection’ protocols
Fig.: Schematic representation of the analytical protocol: (a) binding of the target to the
magnetic beads; (b) hybridization with the CdS-labeled probe; (c) dissolution of CdS tag;
(d) potentiometric stripping detection at a mercury film electrode
 The electrical signal for the DNA analysis was proportional
to the cadmium signals of oxidizing metal Cd0 to Cd+2
 The magnetic separation with magnetic particles can amplify
the sensitivity of the electrochemical stripping method
 Further amplification of the sensitivity of the DNA
electrochemical detection can be achieved by catalytic
precipitation of cadmium to produce larger particles or
using CNT - loaded CdS tags

20.  The detection method is simple, and uses a lower sample volume based on screen -printed electrodes
as working electrodes and a hand - held potentiostatic device as a measuring system.
 Compared with the conventional CdS/QDs - based procedures, the CNT -based protocol exhibits a
substantially (40 times) larger cadmium signal for the significantly (50 - fold) lower detection
concentration
 Estimated detection limit can: 20 ng ml-1
 Furthermore 500 - fold lowering of the detection limit compared to 20 ng ml-1 is observed with a CNT
carrier
A large enhancement (greater than 12 - fold) of the
cadmium hybridization signal is observed by using
a cadmium catalytic precipitation step
Fig.: Stripping potentiograms for 0.4 mg l-1 target,
without (A) and with (B) catalytic enhancement with 30
µl of 10 mM Cd(NO3)2 and 30 µl of 0.05 M hydroquinone

21.  Simultaneous detection of multiple DNA, based on various semiconductor nanoparticle (QDs)
compositions, has also been developed by Wang et al.
 The stripping voltammogram of these three semiconductor
nanoparticles yields well - resolved and non overlapping
peaks, which allowed simultaneous analysis of three
different kinds of DNA
 This method was further developed with four different
magnetic beads for coding single - nucleotide
polymorphisms using four different encoding QDs (ZnS,
CdS, PbS, CuS)
Fig.: Multitarget electrical DNA detection protocol based
on different inorganic colloid nano-crystal tracers
(a) Introduction of probe-modified magnetic beads
(b) Hybridization with the DNA targets
(c) Second hybridization with the QD - labeled probes.
(d) Dissolution of QDs and electrochemical detection.
Fig.: Typical stripping (A) Response
for a solution containing dissolved
ZnS, CdS, and PbS nanoparticles
(B,C,D) Hybridization stripping
response for three different DNA
targets (T1, T2, T3, respectively ) (E)
Response for a mixture containing the
three DNA targets (F) Stripping
hybridization signals for increasing
concentration of the two DNA targets
(T2 and T3) in 13.5 nM steps (A-E)

22.  The strategy of QDs as labels for the determination of protein is similar to DNA
Fig.: An example of a CdS QD - based
electrochemical immunoassay system for protein
Magnetic bead - based
sandwich immunoassay
Cadmium released
with 1 M HCl
Square - wave voltammetry
measurement of the released Cd2+ ion
Interleukin (IL)
 This QD - based electrochemical immunoassay shows high linearity over the range of 0.5 – 50 ng ml-1
and detection limit of 0.3 ng ml-1
 Liu et al. further developed a disposable
electrochemical immunosensor diagnosis device
(DEIDD) based on QD labels and an
immunochromatographic strip
 This device has been successfully applied for
the detection of prostate - specific antigen in
human serum samples with a detection limit of
20 pg ml−1
#Linear range =0.1–10 ng ml−1 IgG
#Estimated detection limit=30 pg ml−1
Fig. : (A) Schematic illustration of DEIDD; (B) photograph of DEIDD

23.  Wang et al. s reported the QD barcode made by the reverse micelle synthesis method for the
electrochemical determination of a cancer marker ( carcinoembryonic antigen ( CEA ))
 The electrochemical stripping response of the dissolved CdS/PbS barcode shows a linear range for the
logarithmic concentration of CEA from 0.01 to 80 ng ml−1, with an estimated detection limit of 3.3 pg ml−1
Mixed monolayer of thiolated
aptamers on the gold substrate
with the bound protein – QD
conjugates
Sample addition and
displacement of the
tagged proteins
Dissolution of the
remaining captured
nanocrystals
Electrochemical stripping
detection at a coated GC
electrode

24.  Biosensing methods based on QD bioconjugates proved to be successful
 rapid detection of pathogens
 significant improvements are achieved in early cancer diagnostic
 non-conventional therapy of cancer and neurodegenerative diseases
Real Application
in

25. Conclusions
 This chapter has highlighted the progress in the development of electrochemical
biosensors based on QDs and their typical applications
 QDs not only achieve direct electron transfer between the redox protein and
electrodes, but also maintain the bioactivity of redox proteins, thereby improving
the sensitivity and linear range of detection
 QDs as the labels for detection of proteins and DNA can amplify the
electrochemical detection, and the use of different QDs enables the parallel analysis
of different targets
 Electrochemical biosensors based on QDs show great promise for the fabrication of
the third generation of mediator – free biosensors

26. References