SENIOR SECURED PROMISSORY NOTE

Exhibit 4.1

FOR VALUE RECEIVED, (i) ADVANCE NANOTECH, INC., a Delaware corporation (hereinafter called “Borrower” or the “Company”), hereby promises to pay to _____________________, or its assigns (the “Holder”), without demand, the principal amount of ___________________________ ($_____________.00) on October 15, 2009 (the “Maturity Date”), if not paid sooner, and (ii) OWLSTONE NANOTECH INC., a Delaware corporation (hereinafter called “Owlstone”), hereby promises to pay to Holder, without demand all interest accruing on this Note, as provided herein, from the Issue Date through the Maturity Date.

This Note is one of the Notes (the “Notes”) issued by the Company pursuant to the Subscription Agreement dated as of September 18, 2009 (the “Subscription Agreement”) by and among the Company and each of the Subscribers named in Exhibit A thereto.  Certain rights under this Note are subject to action or inaction by the holders of 60% of the principal amount of the Notes issued and outstanding (the “Noteholders’ Supermajority”).  Unless otherwise separately defined herein, all capitalized terms used in this Note shall have the same meaning as is set forth in the Subscription Agreement.

ARTICLE I

GENERAL PROVISIONS

1.1            Payment Grace Period .  The Borrower and Owlstone shall have a ten (10) business day grace period to pay any amounts due under this Note, after which grace period and during the pendency of an Event of Default (as defined in Article III) a default interest rate of eleven percent (11%) per year shall apply to the amounts owed hereunder.

1.2.            Interest Rate .

(a)           Interest payable on this Note shall accrue at the rate of at the rate of three percent (3%) per month. Accrued interest shall be paid in full on the Maturity Date.

(b)           All accrued interest will be payable in shares of common stock, par value $0.001 per share, of Owlstone Nanotech, Inc., a Delaware corporation (“Owlstone Common Stock”).  The number of shares of Owlstone Common Stock to be issued shall be determined by dividing the aggregate accrued interest on the Notes by $0.25.

1.3.            Payment in Full on Maturity Date .  This Note shall be payable in full on the Maturity Date; provided, however, that the Borrower retains the right to prepay amounts under this Note without penalty.

1.4.            Extension of the Maturity Date .  Upon request of the Company and consent of the Noteholders’ Supermajority, the Maturity Date may be extended for a period of up to six (6) months.

ARTICLE II

CONVERSION

2.1            Conversion Options .  Upon satisfaction of the conditions set forth in Section 2.2 hereof, the principal on this Note shall automatically be converted to shares of Common Stock at a conversion price equal to $0.25 per share if (i) the Holder elects to have the Note so converted by delivering notice to the Company, (ii) the Company elects to have the Note so converted by delivering notice to the Holder or (iii) the Noteholders’ Supermajority elects to have all of the Notes so converted by delivering notice to the Company and the other holders of Notes.  Notwithstanding the foregoing, the conversion options shall be deferred until such time as the Company has taken such action as may be necessary to increase the number of shares of Common Stock the Company is authorized to issue.  If the foregoing conversion of the principal on this Note occurs, Owlstone shall also simultaneously issue shares of Owlstone Common Stock for the accrued interest thereon.

2.2.            Conditions to be Met .  The right of the Company to cause the Notes to be converted to shares of Common Stock shall not arise unless and until the following conditions have been satisfied:

(a)           The holders of all of the outstanding Senior Secured Convertible Notes issued by the Company during the calendar years 2007 and 2008 shall have executed and delivered to the Company a Waiver giving up all rights with respect to the price protection anti-dilution provisions in such notes and  attached warrants, and have agreed to convert said notes to equity subject to the Holders conditions having been satisfied; and

(b)           New capital in the amount of $5,000,000 shall have been raised by the Company through either the issuance of new notes, equity, or direct investment into the company; or

(c)           Noteholders’ Supermajority agree that the Company has reached a level of sustainable profitability, as defined by positive cash flow for at least two quarters.

2.3.            Issuance of Shares .  Shares of Common Stock acquired upon the conversion of this Note shall be deemed to have been issued to the Holder as of the date of the notice to the Company or the Holder, as the case may be.  Any certificates for the shares of Common Stock and Owlstone Common Stock so acquired shall be delivered to the Holder within a reasonable time, not to exceed four (4) business days, after receipt by the Company of this Note from the Holder, and such certificates shall be accompanied by payment of any amount in lieu of the issuance of a fractional share.

2.4.            Taxes on Issuance .  The issuance of shares of Common Stock upon conversion of this Note and the issuance of shares of Owlstone Common Stock as payment of interest on this Note shall be made without charge to the Holder, including, without limitation, any charge for any tax which may be payable in respect thereof, and such shares shall be issued in the name of, or in such names as may be directed by, the Holder; provided , however, that neither the Company nor Owlstone shall be required to pay any tax which may be payable in respect of any transfer involved in the issuance and delivery of any such shares in a name other than that of the Holder, and neither the Company nor Owlstone shall be required to issue or deliver any such certificate unless and