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Supplementary Materials
1. Detailed analysis of 3-stage ring oscillator circuit and the time-domain response of highpass filter (corresponding to Figure 2 of the manuscript).
FIG.1S. This figure shows the waveforms of V 1,V2 and V3 in ring oscillator circuit when Cvar1 is 30pF. The step
transition of V1 is coupled into V3 through Cvar1 by a step of ±ΔV. Then V3 is either charged or discharged to half the
power supply where the inverter U3 toggles. V1 experiences the reverse of its state and the similar process takes place
again. The overall frequency of the circuit is determined by the delay produced in the second inverter stage.
In Figure 1S, the step amplitude, ΔV must be modeled to analytically explain the ring oscillator
circuit. In the first step the total parasitic capacitor at V3 needs to be calculated. The probe (Tektronix
063087005) presents a 8pF parasitic capacitor, Cprob, according to the datasheet. However the exact value of
Cx is not available from the datasheet. To determine this value, write down the equation relating the step
input at V1 and the step output at V3 at the instant when U1 toggles
V3 
V1  Cvar
Cvar  C prob  C x
(1)
substitute the value of ΔV1 (5 V), ΔV3 (3.6 V), Cvar (30 pF) and Cprob (8 pF), the estimated Cx is 3.6 pF.
When the probe is removed (Cprob equals 0), use the estimated Cx and equation 1, the calculated real step
output at V3 is 4.5 V, corresponding to the case Cvar is 30 pF. When Cvar is 6.5 pF this value is 1.4 V.
FIG.2S. The equivalent circuit after U1 and U2 toggle is used to calculate the time domain response of V 3.
The variation of V3 after this point can be analyzed by Figure 2S. The currents flows through
resistor R1 and two capacitors, Cvar and Cx, can be related by writing down the KCL equation
V3 (t )
d (5  V3 (t ))
d (0  V3 (t ))
 Cvar
 Cx
R1
dt
dt
(2)
solve this first order linear homogeneous differential equation, the response of V 3(t) is
V3 (t )  V3 (0 ) exp[ 
t
R1 (Cvar  C x )
]
(3)
The delay created by the second stage from the positive edge of V 1 is
Td  R1 (Cvar  C x ) ln(1 
2V3
)
(4)
Vdd
so the frequency of the oscillator output, CLK, by ignoring the delay provided by other stages, follows
fCLK 
1
2Td

1
2V
3)
2(Cvar  C x ) ln(1 
Vdd
(5)
Substituting Cvar (30pF), Cx (3.6pF) and ΔV(4.5V) into equation (5), the calculated frequency as C x equals
30pF is 195.3 kHz, and the measured value was 212.2kHz. Similarly, by replacing C var with 6.5pF and ΔV
with 1.4V, the calculated frequency when Cx is 6.5pF is 1.504 MHz, and the measured value was 1.494
MHz. For both cases the error is below 8%.
FIG.3S. The S domain equivalent circuit is used to analyze the time-domain response of the high-pass filter. (a) In time
domain schematic, the output of inverter Ua1 (Ub1) in figure 2 is replaced by the switch S. The initial conditions for Cvar
and the output, y(t), are 0V and 5V. (b) In the equivalent S domain circuit after 0 + time point, the capacitor and voltage
source are replaced with their corresponding S domain model. (c) In time domain solution of high pass filter output, the
FWHM can be evaluated by calculating the time spent from the 0 + to the point that y(t) reaches half the voltage supply.
The analysis of the FWHW of the pulse generated from the R-C high-pass filter is based on the S
domain model of the high-pass filter shown in figure 3S(b). The output after 0 - time point follows
Y (s) 
U (s)
1  sRC var
(6)
Substituting U(s) and taking the inverse Laplace transform, the time domain response of the circuit (t>0 -) is
t
t
RC
y (t )  5u (t )  5e
u (t )  5(1  e  )u (t )
(7)
Consider the fact that y(t)=5 for t<0, the total response is
t
y (t )  5  5e RC u (t )
(8)
The theoretical FWHM of the output can be written as
t
FWHM
 RC ln 2  0.69
(9)
2. Optical properties of ZnSe Qdot based DNA sensors
Figure 4S compares the fluorescence emission spectra of three samples of ZnSe Qdots that were dispersed
in PBS buffer and correspond to the following structures: (1) ZnSe Qdots capped with a layer of 11mercaptoundecanoic acid (MUA), (2) ZnSe Qdots capped with MUA and each conjugated to one molecule
of dC25 ssDNA, and (3) ZnSe Qdots capped with MUA and each conjugated to one molecule of
dC25/dG25 dsDNA resulting from hybridization of the conjugated dC25 ssDNA with free complementary
dG25 ssDNA target. Figure 5S shows the observed peak emission intensity of the samples corresponding
to Figure 4S. Figure 6S shows the absorption spectrum of MUA-capped ZnSe Qdots.
FIG.4S. Fluorescence emission spectra of MUA-capped (1) ZnSe Qdots, (2) Qdot-ssDNA conjugates, and (3) QdotdsDNA conjugates dispersed in PBS buffer.
FIG.5S. Peak emission intensity of MUA capped ZnSe Qdots, Qdot-ssDNA conjugates, and Qdot-dsDNA conjugates
dispersed in PBS buffer, whose fluorescence emission spectra are shown in Figure 4S.
FIG.6S. Absorption spectrum of MUA capped ZnSe Qdots dispersed in PBS buffer.