SCHOOL OF ENGINEERING
BIOSENSORS AND INSTRUMENTATION (MSC)
PGEE11040
Exam Diet: April/May 2020 Duration: 48 hours Expected workload: Two Hours plus upload
Exam starts: 13:00 on 12/05/2020 Exam ends: 13:00 on 14/05/2020 All times are BST (UTC+1)
Before commencing work, please read the academic, formatting, scanning and uploading guidance.
Examination information
• This paper consists of TWO sections.
• Candidates should attempt THREE questions, chosen as follows:
• Section A: ONE question. Attempt the whole section.
• Section B: Answer TWO out of the THREE questions. Only TWO out of three questions will be
marked. Candidates must indicate which two questions are to be marked if three questions have been
answered. If no indication is provided, the examiner will mark the FIRST TWO questions answered.
Specific instructions
• Students should assume reasonable values for any data not given in a question, or not available on a
datasheet, and should make any such assumption clear on their answer sheets.
• Students in any doubt as to the interpretation of the wording of a question, should make their own decision,
and should state it clearly on their answer sheet.
• Write concise, complete answers. If a length limit is given, stay within it. Produce equations and diagrams to a
good hand-drawn standard.
• This is an open book exam. This means you can freely access any printed or online materials to aid you in
your answers. Online materials can include text, images, videos and data. You may NOT engage in
interactions or discussions relating to the exam questions or examined subject matter in any form. Sharing
the answers to this exam in any way, by any means and in any form is STRICTLY NOT allowed.
• Use only a standard calculator. Do not use computer-based spreadsheets, mathematical solvers, simulation
tools, graphing calculators or any other tool which is interactive in nature, such as online mathematical
equation solvers.
Technical instructions (For full details see the formatting guidance and how to upload your exam to learn)
• Write in dark blue or black ink on white or light-coloured A4 paper, or the nearest equivalent size; unlined,
lined, and graph paper are all acceptable, as are pages with holes for binding.
• Write on one side of the paper only. Start every question on a new page. Use portrait orientation.
• On every page, rule a horizontal line 2cm below the top. Clearly write above the line, leaving large spaces
between characters, the QUESTION NUMBER (left side) and your EXAMINATION NUMBER (right side).
• Arrange your pages so that your answers are given in the same order as the question paper.
• Take clear individual pictures of each page and combine in a single PDF document according to the scanning
instructions.
• Name your file with the course code and your examination number, e.g. ENGI00000-B123456.pdf
• Check your file carefully then upload it in the ONLINE EXAM area for this course on LEARN.
• If you require technical support, contact [email protected]
Special Items
None.
Convenor of Board of Examiners: Dr A Hamilton
External Examiner: Professor R Stewart
NOTE: Answers to questions in sections A and B should
typically require no more than four pages. Complete and
concise answers will be rewarded accordingly. Answers
that contain excessive superfluous material may be
penalised.
SECTION A
Question A1
a) Biosensors are able to achieve highly specific recognition of analytes but at
the same time they can be strongly affected by environmental factors.
Suggest two effects that can result in poor selectivity or interference in a
biosensor system, and for each suggest a method to reduce the negative
impact on the measurement. (4)
b) (i) Explain what is meant by the bandwidth of a sensor and what
aspects of the sensor design may determine the limits of this
parameter. (3)
(ii) Give two reasons why it might be useful to apply filtering to reduce
the bandwidth of the signal from a sensor. (2)
c) The first stage in analogue sensor instrumentation is often an amplifier.
Suggest two desirable characteristics of a voltage amplifier for a sensor,
giving reasons for your answers. (4)
d) Consider a potentiometric pH sensor which is calibrated by placing it in a
buffer solution with a pH of 10, where it gives an output potential of +0.3 V
and then in a buffer with a pH of 3 which gives an output of +0.72 V
(i) Assuming that the sensor output is linear over a range of pH from 2
to 12, what is the sensitivity in this range? (1)
(ii) Assume that the sensor will be connected to the input of a 10 bit
analogue to digital converter (ADC) with an input range of 0 V to
5V. Describe, with suitable circuit diagram, the design of an amplifier
which can be used to connect the sensor to the ADC. (4)
(iii) What is the resolution of the digitised pH reading? (2)
PGEE11041 BioSensors and Instrumentation – May 2020
SECTION B
Question B1
a) A three electrode electrochemical sensor can be represented by the
equivalent circuit shown in Figure QB1a. Explain what each component
represents in terms of the physical structure and electrochemical
phenomena of the experimental setup. (6)
b) Figure QB1b shows a standard potentiostat circuit, refer to this for the
following questions.
(i) Describe how the potentiostat can be used to control an
electrochemical reaction at the Working Electrode (WE) with
reference to the input V1 and the operational amplifiers OA-1 and
OA-2. (4)
(ii) Describe how the current through the working electrode is measured
providing an equation for the appropriate output from the circuit. (2)
(iii) It is well known that stability problems may result from the current
measurement circuit when a high gain is used. With a suitable circuit
diagram, describe an alternative form of potentiostat circuit where
the working electrode is connected directly to ground, and state how
the current is measured. (3)
(iv) Referring to your answer for part b)(iii), describe an amplifier circuit
which is suitable to measure the working electrode current. Suggest
values for components in the current measurement circuit to give
a maximum output voltage of 5 V when it is used in a biosensor
application where the sensor has the output characteristic shown in
Figure QB1c. (5)
RE
WECE
Cref Rref
Rs Ru
Rct
CdlCdl
Rct
Figure QB1a:
CONTINUED OVER
PGEE11041 BioSensors and Instrumentation – May 2020
Figure QB1b:
PGEE11040 Biosensors and Instrumentation – April/May 2013

Figure QB3b




Figure QB3c

END OF PAPER
WE
RE
CE

+

+

+ OA-3
OA-1
OA-2
R
R
Rout
ein
iw
-iwRout
-e (vs. ref) = -e wk in
Analyte Concentration (AU)
Fig re QB1c:
PGEE11041 BioSensors and Instrumentation – May 2020
Question B2
a) For each of the micromechanical sensing methods shown below, explain the
transduction mechanism in detail.
(i) Capacitive; (2)
(ii) Piezoelectric; (2)
(iii) Piezoresistive; (2)
b) Figure QB2b illustrates an immunosensor based on a cantilever which
bends up due to the mechanical forces developed when the probe antibody
attached to the top surface binds with its specific antibody. The bending can
be sensed using thin-film metal strain gauges fabricated on the top surface
of the cantilever which have the following characteristics:
• Gauge factor, GF = 1.9;
• Unstrained resistance, Rg = 1 k⌦.
The equation for the gauge factor, where R is the change in strain sensor
resistance, is:
GF =
R
"Rg
(i) Assume that the maximum strain in the gauge attached to the top
surface is " = 1.5 %. Describe in detail a passive circuit that can
be used to transduce the strain into a voltage output. This should
be compensated for changes in temperature. Your answer should
include a calculation of the output voltage. (6)
(ii) Strain gauges can also be microfabricated as diffused resistors in
a semiconductor such as silicon. State whether you would expect
these to have a higher or lower sensitivity than a metal gauge and
explain your answer. (2)
(iii) A new design for the biosensor uses diffused resistors in a silicon
cantilever. If the resistors increase in resistance when strained
longitudinally and decrease when strained transversely explain how
you would alter your previous circuit to maximise the output. (2)
CONTINUED OVER
PGEE11041 BioSensors and Instrumentation – May 2020
c) An alternative method of sensing the antibody-antigen reaction described
in part b) is to detect the increase in mass when the antigen binds to the
antibody.
(i) Describe how capacitive and piezoelectric mechanical transduction
methods could be used to sense this change in mass. (2)
(ii) Assume that the sensor will operate in a liquid environment. State
which transduction method would be best suited and explain your
answer. (2)







Figure QB2b:
PGEE11041 BioSensors and Instrumentation – May 2020
Question B3
a) Figure QB3a shows a schematic cross section through an ion sensitive
field effect transistor (ISFET) designed for integration into a CMOS
(complementary metal oxide semiconductor) microfabrication process.
(i) State which part of the structure is sensitive to the solution pH. (1)
(ii) Explain how this differs from the original implementation of the
ISFET and give a reason why the original structure was not CMOS
compatible. (3)
(iii) In the site binding model of an ISFET the pH sensitive surface is
made up of hydroxyl groups which can interact with the measured
solution with the following equilibrium reactions:
SiOH SiO– + H+
SiOH +2 SiOH + H
+
With the aid of an appropriate diagram, describe the interactions
between the measured solution and SiOH groups at the surface of
the ISFET, as the pH changes. This should include a description of
the effect of the pH on the threshold voltage of the ISFET. (4)
(iv) Figure QB3b shows the equivalent circuit of an extended gate ISFET
with the addition of a reference electrode to control the solution
potential. Explain the physical source of the capacitances Cd and
CH and describe how they are affected by the composition of the
solution being measured. (3)
(v) Again with reference to Figure QB3b, state where the pH dependent
potential difference VpH will be dropped (1)
b) Sensors based on semiconducting nanowires or nanoribbons can be applied
in biosensing in a similar way to the ISFET/CHEMFET.
(i) Describe a possible advantage of a semiconducting nanowire
sensor over an ISFET based sensor. (2)
(ii) Describe the fabrication of a single crystal silicon nanowire sensor,
which does not use advanced lithographic techniques. (2)
CONTINUED OVER
PGEE11041 BioSensors and Instrumentation – May 2020
(iii) Describe the application of a semiconducting nanowire device to
sense a biological recognition event. Your answer should include
details of the specific biochemical interaction involved, the method of
transduction used to obtain a quantifiable output of the concentration
of the analyte, and any possible problems with the measurement. (4)
















Figure QB3a:
S
D
VG
Cd CH Cpass
Solution Device
floating
gate
VRE
Reference
electrode
Figure QB3b:
END OF PAPER
PGEE11041 BioSensors and Instrumentation – May 2020

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