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                                                                   EXHIBIT 10.22

                          OPTICAL SENSORS INCORPORATED
                           CONVERTIBLE PROMISSORY NOTE

$150,000.00                                                        March 8, 2001
                                                          Minneapolis, Minnesota

     Optical Sensors Incorporated, a Delaware corporation (the "Company"), for
value received, hereby promises to pay to Circle F Ventures LLC, or any person
to whom this Note is subsequently transferred and who becomes a registered
holder of this Note (the "Holder"), at the Holder's office or to any account
designated by the Holder to the Company in writing, the principal sum of one
hundred fifty thousand Dollars ($ 150,000.00), or such lesser amount as may be
advanced hereunder from time to time, on the first anniversary date of this Note
(the "Maturity Date"). This Note shall not bear interest.

     This Note is issued pursuant to the Bridge Loan Agreement, dated as of
March 7, 2001, by and among the Company and the Holder (the "Agreement"). This
Note is subject to the terms and condition of the Agreement, which is hereby
incorporated by reference.

     The Company waives demand, presentment, protest, notice of dishonor and any
other form of notice, not expressly required by the Agreement, that may be
required to hold the Company liable hereunder.

     This Note is subject to the following terms and conditions:

1. Payment. After April 30, 2001 this Note may be prepaid in whole or in part at
any time or from time to time, on ten (10) days' prior written notice to the
Holder, without premium or penalty.

2. Conversion.

     2.1 Terms of Conversion. The Holder shall have the right to convert all or
any portion of principal balance and accrued interest under this Note, at the
option of the Holder, into shares of the Company's common stock, $.01 par value
(the "Common Stock") at any time, and from time to time. If the Company
completes a private placement of equity securities (the "Financing") by April
30, 2001, the conversion price of this Note shall be equal to the per share
price of the securities sold in the Financing. If the Company does not complete
the Financing by April 30, 2001, the conversion price of this Note shall be $.25
per share. The conversion price is subject to adjustment pursuant to Section 2.4
below. Upon conversion, this Note must be surrendered and accompanied by a
written conversion notice (hereinafter referred to as the "Conversion Notice")
delivered to the Company at its principal office during usual business hours.

     2.2 Conversion Securities. The securities issuable upon conversion of this
Note pursuant to this Article 2 are referred to herein as the "Conversion
Securities."

     2.3 Issuance of Common Stock. The conversion of this Note will be deemed to
have been made at the close of business on the date on which this Note has been
surrendered for conversion with the Conversion Notice duly executed (the
"Conversion Date"). As of the Conversion Date, the rights of the Holder as a
noteholder with respect to that portion of this Note
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which is so converted will cease and the Holder will be treated for all purposes
as having become the record holder or holders of the Conversion Securities as of
such Conversion Date. No fractional shares of Conversion Securities will be
issued upon the conversion of this Note, but, instead of any fraction of a share
which would otherwise be issuable, the Company will deliver an amount of cash
equal to such fraction multiplied by the then applicable conversion price.

     2.4 Adjustment of Conversion Price for Optional Conversion. The conversion
price shall be subject to adjustment from time to time as follows:

          (a) In case the Company shall subdivide its outstanding shares of
     Common Stock into a greater number of shares at any time, the then
     applicable conversion price in effect immediately prior to such subdivision
     shall be proportionately reduced, and conversely, in case the outstanding
     shares of Common Stock of the Company shall be combined into a smaller
     number of shares, the then applicable conversion price in effect
     immediately prior to such combination shall be proportionately increased.

          (b) If any capital reorganization or reclassification of the capital
     stock of the Company, or consolidation or merger of the Company with
     another corporation, or the sale of all or substantially all of its assets
     to another corporation shall be effected in such a way that holders of
     Common Stock shall be entitled to receive stock, securities or assets with
     respect to or in exchange for Common Stock, then, as a condition of such
     reorganization, reclassification, consolidation, merger or sale, lawful and
     adequate provision shall be made whereby the Holder shall thereafter have
     the right to receive upon the basis and upon the terms and conditions
     specified herein and in lieu of the Conversion Securities immediately
     theretofore receivable upon the conversion of this Note, such shares of
     stock, securities or assets as may be issued or payable with respect to or
     in exchange for a number of outstanding shares of such Common Stock equal
     to the number theretofore receivable upon the conversion of this Note had
     such reorganization, reclassification, consolidation, merger or sale not
     taken place, and in any such case appropriate provision shall be made with
     respect to the rights and interests of the Holder to the end that the
     provisions hereof (including, without limitation, provisions for
     adjustments of the then applicable conversion price and of the number of
     securities receivable upon the conversion of this Note) shall thereafter be
     applicable, as nearly as may be practicable, in relation to any shares of
     stock, securities or assets thereafter receivable upon the conversion of
     this Note.

          (c) In any case in which this Section 2.4 shall require that an
     adjustment shall become effective immediately after a record date for an
     event, and if the Holder should convert after such record date and before
     the occurrence of such event, then the Company may defer until the
     occurrence of such event (i) issuing the additional shares of Common Stock
     issuable upon such conversion by reason of the adjustment required by such
     event over and above the shares issuable upon such conversion before giving
     effect to such adjustment and (ii) paying to the Holder any amount of cash
     in lieu of a fractional share pursuant to Section 2.3 above.

          (d) All calculations under this Article 2 shall be made to the nearest
     cent or to the nearest one-hundredth of a share, as the case may be.

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          (e) Upon any adjustment of the conversion price, then and in each such
     case, the Company shall give written notice thereof, by first-class mail,
     postage prepaid, addressed to the Holder, at the address of the Holder as
     shown on the books of the Company, which notice shall state the conversion
     price resulting from such adjustment and the increase or decrease, if any,
     in the number of Conversion Securities issuable upon conversion of this
     Note at such price, setting forth in reasonable detail the method of
     calculation and the facts upon which such calculation is based.

          (f) The Company shall give the Holder 20 days' written notice prior to
     the effective date of any of the events described in Sections 2.4(a) or
     2.4(b) above.

     2.5 Adjustment of Number of Shares. Upon each adjustment of the conversion
price pursuant to Section 2.4, the number of Conversion Securities shall be
adjusted by dividing the then unpaid principal amount hereof by the applicable
per share conversion price in effect immediately following such adjustment.

     2.6 Covenants of Company. The Company covenants that all of the Conversion
Securities will, upon issuance, be duly authorized and issued, fully paid,
nonassessable and free from all taxes, liens and charges with respect to the
issue thereof. The Company further covenants that during the period within which
this Note may be converted, the Company will at all times have authorized, and
reserved free of preemptive or other rights for the purpose of issue, such
number of shares of Conversion Securities as shall then be issuable upon
conversion of this Note as herein provided.

3. Consolidation, Merger, Sale or Conveyance

     3.1 Generally. Nothing contained in this Note will prevent any
consolidation or merger of the Company with or into any other corporation or
corporations or successive consolidations or mergers in which the Company or its
successor or successors is a party or parties, or will prevent any sale or
conveyance of the property of the Company as an entirety or substantially as an
entirety to any other corporation authorized to acquire and operate the same.
However, the Company hereby covenants and agrees that any such consolidation,
merger, sale or conveyance will be upon the condition that (a) immediately after
such consolidation, merger, sale or conveyance the corporation (whether the
Company or such other corporation) formed by or surviving any such consolidation
or merger, or to which such sale or conveyance will have been made, will not be
in default in the performance or observance of any of the terms, covenants and
conditions of this Note to be kept or performed by the Company; and (b) the
corporation (whether the Company or such other corporation) formed by or
surviving any such consolidation or merger, or to which such sale or conveyance
will have been made, will expressly assume the due and punctual payment of the
principal of this Note, according to the terms of this Note, and the faithful
performance and observance of all of the covenants, conditions, and requirements
of this Note to be performed by the Company by a supplemental instrument
executed and delivered to the Holder by such corporation.

     3.2 Release; Liability of Successor Corporation. In case of any such
consolidation, merger, sale or conveyance, and upon the assumption by any
successor corporation pursuant to Section 3.1 above, such successor corporation
will succeed to and be substituted for the Company, with the same effect as if
it had been named in this Note in the Company's place, and the Company
(including any intervening successor to the Company which has become obligated

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under this Note) will be relieved of any further obligation under this Note. All
of the covenants, stipulations, promises, and agreements contained in this Note
by or on behalf of the Company will bind its successors and assigns, whether so
expressed or not.

4. Default

     4.1 Events of Default. An "Event of Default" will be deemed to occur upon
the happening of any of the following: (a) the failure to pay when due any
amount of principal or interest payable hereunder, (b) the filing against the
Company which is not dismissed within 60 days thereafter, or by the Company, of
a petition in bankruptcy or for an arrangement or reorganization, (c) the making
by the Company of a general assignment for the benefit of creditors, (d) the
appointment of a receiver or trustee for the Company, (e) the institution of
liquidation or dissolution or reorganization proceedings with respect to the
Company, (f) the Company becoming unable or admitting in writing an inability to
pay its debts generally as they become due, or (g) the occurrence of any breach
by the Company of any representation, warranty or covenant of the Company under
the Agreement, which is not cured within thirty (30) days of written notice from
the Holder.

     4.2 Rights on Default. If an Event of Default occurs and is continuing, the
Holder may declare the principal of this Note, together with any accrued and
unpaid interest, if not already due, to be due and payable immediately, by
written notice to the Company; provided, however, that all amounts due under
this Note shall be automatically due and payable, without any action of the
Holder, upon an Event of Default pursuant to Sections 4.1(b) - 4.1(f) above.
Upon any such declaration, such principal and interest will become due and
payable immediately, anything contained in this Note to the contrary
notwithstanding.

     4.3 Enforcement. If the principal of this Note, together with all accrued
and unpaid interest on this Note, becomes due and payable immediately, whether
by declaration of the Holder or automatically, the Holder may proceed to protect
and enforce its rights by an action at law, suit in equity, or other appropriate
proceeding. The Company shall pay all costs and expenses of collection,
including, without limitation, attorneys' fees and disbursements in the event
that any action, suit or proceeding shall be brought by the Holder hereof to
collect this Note.

5. Miscellaneous.

     5.1 Headings. The headings in this Note are inserted for convenience only
and will not affect the meaning or interpretation of all or any part of this
Note.

     5.2 Governing Law. This Note will be deemed to be a contract made under the
laws of the State of Minnesota, and for all purposes will be construed in
accordance with the laws of the State of Minnesota.

     5.3 Construction. Wherever possible, each provision of this Note will be
interpreted in such a manner as to be effective and valid under applicable law,
but if any provision of this Note is prohibited by or invalid under applicable
law, such provision will be ineffective only to the extent of such prohibition
or invalidity without invalidating the remainder of such provision or the
remaining provisions of this Note.

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     5.4 Amendments. This Note may not be and will not be deemed or construed to
have been modified, amended, rescinded, canceled or waived, in whole or in part,
except by a written instrument signed by the Company and the Holder.

     5.5 Payment Date. In case the Maturity Date or the date fixed for
prepayment of this Note is not a business day, then payment of principal and
interest to the Holder need not be made on such date, but may be made on the
next succeeding business day with the same force and effect as if made on the
Maturity Date or the date fixed for prepayment and no interest will accrue for
the period after such date.

     IN WITNESS WHEREOF, the Company has caused this Note to be duly executed as
of the date set forth above.

                                       OPTICAL SENSORS INCORPORATED

                                       By /s/ Paulita LaPlante
                                          --------------------------------------
                                          Paulita LaPlante
                                          President and Chief Executive Officer

                                 ---------------

THIS NOTE HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED,
OR STATE SECURITIES LAWS AND MAY NOT BE SOLD, TRANSFERRED, ASSIGNED, OFFERED,
PLEDGED OR OTHERWISE DISTRIBUTED FOR VALUE UNLESS THERE IS AN EFFECTIVE
REGISTRATION STATEMENT UNDER SUCH ACT AND SUCH LAWS COVERING THIS NOTE OR THE
COMPANY RECEIVES AN OPINION OF COUNSEL ACCEPTABLE TO THE COMPANY STATING THAT
SUCH SALE, TRANSFER, ASSIGNMENT, OFFER, PLEDGE OR OTHER DISTRIBUTION FOR VALUE
IS EXEMPT FROM THE REGISTRATION AND PROSPECTUS DELIVERY REQUIREMENTS OF SUCH ACT
AND SUCH LAWS.

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                                                                   EXHIBIT 10.23

March 10, 2001

Special Situations Fund III, L.P.
153 East 53rd Street, 55th Floor
New York, NY 10022

Gentlemen:

This letter is delivered to you with reference to (i) the Investment Agreement,
dated as of March 10, 2000 (the "Investment Agreement"), by and among Optical
Sensors Incorporated (the "Company"), Circle F Ventures LLC and Special
Situations Fund III, L.P. ("SSF"), (ii) the Convertible Promissory Note ("Note")
issued by the Company to SSF pursuant to the Investment Agreement and (iii) the
Warrants (individually, a "Warrant" and collectively, the "Warrants") issuable
by the Company to SSF upon conversion of the Note.

Section 3(a) of the Investment Agreement and the first sentence of Section 2.1
of the Note are hereby amended to the extent necessary to change the conversion
price of the Note to Twenty Five Thousand Dollars ($25,000) per Unit. Section
3(a) of the Investment Agreement is hereby amended to the extent necessary to
change the initial exercise price of the Warrant to $.50 per share.

Section 1 of the Investment Agreement and the first paragraph of the Note are
hereby amended to the extent necessary to extend the Maturity Date of the Note
to September 30, 2001.

Section 3(c) of the Investment Agreement is hereby amended and restated in its
entirety as follows:

     (c)  Conversion. The entire principal balance of the Note will convert into
          Units, without any action of the Investor, 30 days following written
          notice from the Company, of the earlier to occur of (i) the completion
          of a financing by the Company that results in gross proceeds to the
          Company of at least $2,500,000, (ii) the Company entering into a
          distribution or other strategic relationship with a third party under
          which the Company will receive value of at least $2,000,000 during the
          term of such relationship, or (iii) any combination of the
          transactions described in clauses (i) or (ii) which, in the aggregate,
          results in gross proceeds to the Company or under which the Company
          will receive at least $2,500,000.

The registration rights referred to in Section 7 of the Investment Agreement
shall cover all shares of Common Stock issuable upon conversion of the Notes and
exercise of the Warrants.

This letter shall be binding on the Company and SSF, but shall not amend the
Investment Agreement with respect to any other party thereto or amend the terms
of any notes or warrants
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Special Situations Fund III, L.P.
March 10, 2001
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issued under the Investment Agreement to any party other than SSF. In the event
of any inconsistency between the terms of this letter and the terms of the
Investment Agreement, the Note or any Warrant, the terms of this letter shall
govern.

OPTICAL SENSORS INCORPORATED           SPECIAL SITUATIONS FUND

/s/ Wesley G. Peterson                 /s/ David Greenhouse

Wesley G. Peterson                     David Greenhouse
Chief Financial Officer

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